Early vertical take-off and landing fighters from southern California, part 1: the turboprop-powered tail-sitters

The German invasion of the Soviet Union and the Japanese attack on Pearl Harbor in 1941 became stark reminders of the dangers of airfields being mauled by enemy airstrikes if they were not fully prepared for an enemy attack. Although helicopters were America's premier aircraft optimized for vertical take-off and landing (VTOL) beginning in World War II, the biggest practical limitation of the helicopter is the fact that a helicopter itself has a built-in speed limitation caused by the motion of the rotor blades, meaning that the average helicopter can attain a speed of only 160 miles per hour (260 km/h). As a helicopter flies through the air, one rotor blade is moving away from the other blades, and if a helicopter flies too fast, then a raft of onrushing air will overtake the retreating blade, causing that rotor blade to lose lift and the helicopter to stall from the air, a phenomenon called retreating blade stall. Aware of the constraints on the top speeds of existing helicopters due to the motion of the rotor blades, beginning in the late 1940s the US armed forces decided to look at the idea of combat aircraft designed to combine the vertical lift of helicopters with the combat performance of a fighter plane. The aviation industry in southern California would take the lead in envisaging designs for early in vertical take-off fighter planes, and as the 1950s progressed there would a be a diversity of ideas regarding the best means of vertical takeoff for a fighter. Therefore, I've dedicated the first post in a three-part series on VTOL fighter designs from southern California to tail-sitter designs. 

Top: Sideview drawing of the Northrop N-63 proposal
Bottom: Desktop models of the Lockheed XFV-1 (left) and Northrop N-63 (right) in a model store of the Planes of the Fame Museum (photographed by me)

In 1948, the US Navy announced a requirement for an aircraft capable of vertical takeoff and landing aboard platforms on the afterdecks of conventional ships. By 1950, this requirement had evolved into the OS-122 specification for a research aircraft that could be developed into a ship-based VTOL convoy escort fighter. Convair, Goodyear, Grumman, Lockheed, Martin, and Northrop submitted designs for the OS-122 requirement, all of them powered by a single Allison T40 turboprop engine driving counter-rotating propellers and standing to their tail empennages for VTOL. The Convair Model 5 featured the delta-wing layout of the earlier XF-92 and XF-92A as well as the future F-102 Delta Dagger and F-106 Delta Dart, and it had two large tail fins for support in an upright position. By contrast, the Lockheed  L-200-1 had the straight wings of the P-80/F-80 Shooting Star, T-33 trainer, and F-94 Starfire, and it was designed to stand on four tail fins in upright position. (A number of L-200 designs with backswept wing and canards were also envisaged, and a horizontal take-off and landing version was proposed as the L-210-2, but these were eventually discarded.) The Northrop N-63 had a straight wing with dihedral and a very large ventral T-tail, with provisions for four 20-mm cannons mounted in large pods on the wing tips. The tail-sitter concept, however, was not new in these designs. In the final months of the Third Reich, German aircraft manufacturers Focke-Wulf and Heinkel came up with designs for tail-sitting VTOL fighters, the Focke-Wulf Triebflügel with a three-blade rotor around the center of the aircraft behind the cockpit, powered by three ramjets at the tips of the blades, and Heinkel Lerche and Wespe with ducted propellers. None of these proposals left the design phase before Nazi Germany's surrender, but some design attributes of the wartime German tail-sitter projects, namely the cruciform tail empennage, would eventually find their way into the VTOL fighter designs for the OS-122 requirement.

Models of the Convair XFY (left) and Lockheed XFV (right) at the Planes of Fame Museum

The Navy selected the Convair and Lockheed proposals for full-scale development and on March 31, 1951, a contract for three Convair Model 5 prototypes (BuNos 138648/138650) was issued, and the Convair design became XFY-1, whereas the Lockheed L-200-1 was designated XFO-1 (later changed to XFV-1), with a contract for two prototypes (BuNos 138657/138658) plus a static test article being signed on April 19. The XFY-1 was 34 feet 9.3 in (10.6 meters), with a wingspan of 27 feet 7.9 in (8.43 meters), a wing area of 355 square feet (33.0 square meters), and a horizontal height of 22 feet 1 in (6.73 meters). Compared to the XFY-1, the XFV-1 was a bit larger, measuring 36 feet 8.94 in (11.2 meters) long with a wingspan of 30 feet 10 in (9.4 meters) and a wing area of 246 square feet (22.8 square meters). A full developed production version of the Convair design would be armed with either 20-mm cannons or two separate packs of 2.75 in (70 mm) folding-fin unguided rockets in wingtip pods. A planned production version of the XFV-1, the FV-2, would carry the same type of armament as the XFY-1 but would have the T40 replaced by an Allison T54 turboprop. Because the mode of vertical take-off and landing for which the Lockheed and Convair machines were designed resembled the action of a pogo stick, the XFV-1 and XFY-1 would be collectively nicknamed the "Pogo".

Top: Lockheed XFV-1 in upright position (left) and forward flight (right)
Bottom: Convair XFY-1 during flight testing

The first XFV-1 prototype was completed in the spring of 1953. Due to the T40-A-6 engine offering insufficient power for a vertical takeoff, it was decided to fit the XFV-1 with a temporary fixed-position undercarriage (composed of two forward wheels supported by four struts and downward-facing wheels on the two lower vertical fins) and undertake horizontal takeoffs pending arrival of the more powerful T40-A-14 unit. A special ground-handling rig was also built to allow the aircraft to be moved easily in a horizontal position and tilted upright for take-off. Taxi trials and ground tests of the XFV-1 began in late November 1953, with test pilot Hermann "Fish" Salmon at the controls (hence the XFV-1 being known unofficially as the "Salmon"). During one high-speed taxi test on December 23, 1953, the XFV-1 made a short hop for a few seconds, but it would not be until June 16, 1954 that it made its first official flight at Edwards Air Force Base. A total of 22 flight were conducted until March 15, 1955, with Salmon himself carrying out transitions from horizontal to vertical flight and propeller vibration tests. With the T40-A-14 running into developmental problems, the XFV-1 would never make a vertical take-off or landing during flight testing. The second XFV-1 prototype was nearly completed but not flown, while the static test airframe was not completed. Meanwhile, in early 1954 the second of the three XFY-1 prototypes in order was completed, the first aircraft being partly assembled and relegated to use for engine tests and the third prototype being used for stress tests only. Tethered tests of the XFY-1 at the Moffett Field Naval Air Station near San Francisco began on April 29, 1954, piloted by Convair test pilot James "Skeets" Coleman. The first free flight of the aircraft took place on August 1 at Naval Auxiliary Air Station Brown Field in San Diego, where Coleman made the first successful vertical take-off, transition to forward flight, and vertical landing on November 2, making the XFY the first vertical take-off aircraft other than a rotorcraft to make a VTOL flight. Test flights of the XFY-1 continued until June 16, 1955, and by this time it was apparent that the tail-sitter idea was not going to an operationally practical concept because the pilot had difficulty landing the XFV or XFY, meaning that he had to look over his shoulder to determine the rate of descent and how close he was to the ground. Moreover, the T40 around which the Lockheed and Convair aircraft had been designed was plagued by teething troubles, also bedeviling the Douglas A2D Skyshark and North American A2J Super Savage attack aircraft, Convair P5Y and R3Y Tradewind flying boats, and Republic XF-84H turboprop fighter.

Left: Second Lockheed XFV-1 prototype (BuNo 138658) on display at the Florida Air Museum, Parkland, Florida.
Right: Convair XFY-1 (BuNo 138649) at the Garber Storage Facility of the National Air and Space Museum in Silver Hill, Maryland.

The US Navy cancelled the XFV and XFY programs in 1956 and the two XFV-1 prototypes as well as the only airworthy XFY-1 prototype miraculously survived the breaker's torch and found their way into museums. The first XFV-1 prototype was transported to Hiller Aviation in Palo Alto, California, where its engine was used for ground tests, before being tested to destruction, while the incomplete second XFV-1 was used as a gate guardian at Naval Air Station Los Alamitos for many years before being donated to the Florida Air Museum in Parkland, Florida. The only XFY-1 to fly was initially used as a gate guardian at Naval Air Station Norfolk in Virginia, before being donated to the National Air and Space Museum in 1973, and it currently resides in storage at the NASM's Garber Storage Facility in Silver Hill, Maryland. The US Navy must have made a practically common-sense decision to cancel XFV and XFY programs not just because of difficulties with landing a tail-sitter plane but also the fact that the Lockheed and Convair "Pogos" would never have kept up with the first generation of Soviet supersonic jet fighters due to the type of engine chosen for these aircraft.

References:

Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.

Chong, T., 2016. Flying Wings & Radical Things: Northrop's Secret Aerospace Projects & Concepts 1939-1994. Forest Lake, MN: Specialty Press.

Rose, B., 2013. Vertical Take-off Fighter Aircraft. Hersham, UK: Ian Allan Publishing.

Late-war Douglas successors to the Dauntless and Devastator, part 2: TB2D Skypirate

In my previous post on late-war carrier-based Douglas attack aircraft, I discussed in brief detail the fact that the Douglas TBD Devastator was the US Navy's first monoplane torpedo bomber and the first-ever all-metal aircraft to enter service, but also the first with a completely enclosed cockpit, considering that previous American torpedo bomber aircraft were open-cockpit, all-wood biplanes, but also the fact that the Devastator itself was becoming obsolescent on the eve of the Pearl Harbor attack and had the poor combat performance at the Battle of Midway largely thanks to the technical inadequacies of the Mark 13 torpedo the Devastator was designed to carry. As the TBD Devastator was eventually retired from active combat service after Midway, the Grumman TBF/TBM Avenger took up the combat roles for which the TBD had been designed, playing a pivotal role in the air and sea engagements during battles off the east coast of the Philippines with the Imperial Japanese Navy in late 1944 to early 1945 by sinking the Japanese warships Musashi and Yamato. Although the Avenger formed the backbone of the Navy's torpedo bomber force in the wake of the Battle of Midway, the Battle of Midway was not the end of the road for the development of torpedo bomber aircraft by Douglas. The second and final post of my two-part series on replacements for the Dauntless and Devastator will now concentrate on discussion of Douglas design efforts at creating a successor to the TBD Devastator, which culminated in the XTB2D Skypirate.

Douglas D-295 proposal to the SD-114-6 competition (as of mid-1939) 

In March 1939, the US Navy issued the SD-114-6 requirement for a new three-seat torpedo bomber to replace the TBD Devastator. The crew of this aircraft comprised a pilot, co-pilot/bombardier, and radio operator/gunner, and the proposed aircraft would have a range of 1,000 miles (1,609 km) with 1,500 lb (681 kg) of bombs or one torpedo, a service ceiling of 30,000 feet (9,144 meters), and a gross weight of 12,500 lb (5,670 kg). By August 24, six manufacturers (Brewster, Douglas, Grumman, Hall Aluminum, Vought, and Vultee) had submitted bids for the SD-114-6 competition. The Douglas submission, which bore the company designation D-295, was an improved derivative of the Devastator featuring a single Wright R-2600 Twin Cyclone radial engine, a length of 38 feet 1.75 in (11.63 meters), a wingspan of 47 feet (14.33 meters) (29 feet [8.84 meters] when folded), and armament comprising two 20 mm cannons in the wings, two machine guns on the nose fuselage behind the fuselage, and bombs stowed in the fuselage. In the end, the Navy selected the Grumman and Vought proposals for production, designating them TBF and TBU respectively.

US Navy Bureau of Aeronautics Standard Aircraft Characteristics (SAC) sheet for the Douglas XTB2D-1 Skypirate

Douglas lost the first round in terms of a proposed successor to the TBD Devastator, but it eventually had better luck next time when in February 1942 the US Navy announced a requirement for a large torpedo-scout-bomber. In March 1942, Douglas carried out a series of design studies in response to the VTSB requirement under the designation D-544, culminating in eight different designs, whose weights ranged from 21,000 lb (9,571 kg) to 23,500 lb (10,660 kg). The first three designs featured twin-engine aircraft powered by two Wright R-1820 Cyclone radial engines, and two more had one turbocharged Allison V-3420 liquid-cooled V-cylinder engine in the rear fuselage behind the pilot, the first with a four-blade propeller connected to the engine by a long shaft, and the second with a three-blade propeller ahead of each wing leading edge and connected by angle extension shafts. The last three D-544 design studies were powered by one Pratt & Whitney R-4360 Wasp Major radial engine driving four-blade counter-rotating propellers, of which two had twin vertical stabilizers and the third had a one vertical stabilizer. The Navy's Bureau of Aeronautics eventually opted for the Wasp Major-powered D-544 proposal with a single-tail, and a contract was placed in November for two prototypes (BuNos 036933 and 036934), which were designated XTB2D-1. The design was originally called Devastator II, but later on Douglas changed the aircraft's name to Skypirate.

Top: First XTB2D prototype (BuNo 036933) on the tarmac, early 1945.
Bottom left: First XTB2D-1 prototype (BuNo 036933) landing after a test flight
Bottom right: Second XTB2D-1 (BuNo 036934) in flight 

The XTB2D-1 was 46 feet (14.02 meters) long, with a wingspan of 70 feet (21.34 meters) (36 feet [10.97 meters] when folded), a height of 22 feet 7 in (6.88 meters), a wing area of 605 square feet (56.265 square meters), a gross weight of 34,760 lb (15,767 kg), a loaded weight of 28,545 lb (12,948 kg) with torpedoes, a range of 1,250 miles (2,011 km), a service ceiling of 27,900 feet (8,504 meters), and a top speed of 340 miles per hour (547 km/h). Armament comprised seven 0.50 in (12.70) machine guns and 8,000 lb (3,629 kg) of bombs or four torpedoes. The Skypirate itself was intended to operate from the new Midway-class aircraft carriers as well as the Essex-class carriers. A full-scale mock-up of the XTB2D-1 design was inspected in March and May 1943, and an order was also placed for 23 pre-production aircraft (BuNos 89097/89119). Although the XTB2D-1 had minimal support from the Navy and the Navy itself recommended cancelling the program on May 20, 1944, Douglas slowly proceeded with construction of the Skypirate prototypes. The first of the two XTB2D-1 prototype flew on March 13, 1945, while second prototype followed in the summer of that year, but by this time Japanese forces in the Pacific were collapsing quickly and the Midway-class carriers encountered delays in preparations for entering commission with the US Navy. Hence, the pre-production order for the XTB2D Skypirate was cancelled and test flights of the aircraft were suspended, with both aircraft being scrapped in 1948. Back in November 1944, Douglas had proposed to fit the XTB2D-1 with an auxiliary turbojet in place of dorsal turret, but this scheme did not materialize.

[EDIT: It has come to my attention that a Douglas company projects index lists company designations D-295, D-298, and D-306 as torpedo bomber designs from the first half of 1939. The D-306 is listed in the index as an export variant of the D-298, however, so the D-298 and D-306 are probable company designations for the TBD-1A floatplane version of the TBD Devastator. Since D-295 is listed as being a torpedo bomber and issued in May 1939, two months after the SD-114-6 requirement was issued, it is almost certain that D-295 was the company designation for the Douglas submission for the SD-114-6 specification.]

References:

Buttler, T., and Griffith, A., 2015. American Secret Projects 1: Fighters, Bombers, and Attack Aircraft, 1937 to 1945. Manchester, UK: Crecy Publishing. 

Kowalski, B., 1996. Douglas XTB2D-1 Skypirate. Simi Valley, CA: S. Ginter.

Late-war Douglas successors to the Dauntless and Devastator, part 1: SB2D and BTD Destroyer

During World War II, the El Segundo Division of Douglas built a number of attack aircraft designed to take out Japanese warships in the Pacific with either bombs or torpedoes, including the SBD Dauntless dive bomber and TBD Devastator torpedo-bomber. The SBD Dauntless played a pivotal role in the US Navy victory at the Battle of Midway in June 1942 by sinking the Imperial Japanese Navy's four fleet carriers and heavy cruiser Mikuma, effectively dashing Japan's hopes of conquering the Hawaiian Islands and putting the Japanese military on the defensive by forcing Japan into retreat in the western Pacific. As the Pacific theater of World War II progressed, however, the Curtiss SB2C Helldiver took over the roles that the Dauntless had performed, and the June 1944 Battle of the Philippine Sea would be the last battle involving the SBD. The TBD Devastator was one of the first American monoplane torpedo bombers and the most advanced US Navy attack aircraft at the time of its deployment, designed to pack a devastating wallop against enemy warships when traveling at high speed, but it fared poorly in the Battle of Midway due to defects with the torpedo it was design to carry and lower speed and maneuverability compared to the Mitsubishi Zeroes. I finally got to see the Dauntless in person when I visited the USS Midway carrier museum in San Diego, and the Planes of Fame Museum and Yanks Air Museum in Chino. Although no Devastators are preserved in museums, several TBDs survive intact underwater, pending recovery and possible restoration for eventually display. However, a replica of the TBD Devastator was built for the 2019 film Midway and has since been put on display in the Hangar Section of the USS Midway Museum; I saw the TBD replica during a visit to the carrier museum last June. However, lost in talk of US Navy attack plane development in World War II is the fact that in 1943-1945, Douglas developed a number of new-generation attack aircraft to replace the SBD and TBD. For all purposes and intents, the first post in a two-part discussion about late-war Douglas attack aircraft for the US Navy will center upon the SBD's slated successor, the SB2D/BTD Destroyer.       

Douglas SBD Dauntless (BuNo 28536 shown here, photographed by me at the Planes of Fame Museum in Chino, California in April 2019), the aircraft that the SB2D/BTD Destroyer was designed to replace. 

On February 3, 1941, the US Navy's Bureau of Aeronautics (BuAer) issued a Request for Proposals for a new dive bomber to supplant the Douglas SBD Dauntless and Curtiss SB2C Helldiver. In response, Ed Heinemann, the chief designer at the El Segundo Division of Douglas responsible for design of the Dauntless, envisaged a design for a dive bomber featuring a laminar flow gull-wing, a bomb bay and underwing racks for up to 4,200 lb (1,900 kg) of bombs or one torpedo (typically the Mark 13), and defensive armament comprising two wing-mounted 20 mm (0.79 in) cannon and two remote-controlled turrets, each with two .50 in (12.7 mm) machine guns. For its part, Curtiss' submission was similar in landing gear configuration and size, and it had an internal bomb bay for carrying up to 4,000 pounds (1,800 kg) of bombs, or alternatively, two torpedoes in semi-submerged mountings, plus hardpoints for two 500-pound (230 kg) bombs fitted under the wings, while defensive armament comprised six .50-caliber machine guns or four 20 mm cannon in the wings. Both proposals possessed a tricycle landing gear, a feature not seen in the Navy's operational carrier-based planes, and power was to be supplied by one Wright R-3350 Duplex Cyclone radial engine delivering 2,300 hp (1,700 kW). On June 20, the Navy placed a contract for two prototypes (BuNos 03551/03552) of the Douglas proposal, which was designated XSB2D-1. Meanwhile, the Curtiss design was designated XSB3C-1 and two prototypes (BuNos 03743/03744) were ordered after a mock-up inspection in December, but the Navy eventually judged the Curtiss design inferior to the Douglas aircraft in performance, so the XSB3C program was cancelled in 1942. 

Left: Douglas XSB2D-1 (BuNo 03551) in flight
Right: Douglas BTD-1 Destroyer (BuNo 04963) in flight, July 25, 1944

The first of the two XSB2D-1 prototypes flew on April 8, 1943. Despite being heavier and more complex than the Dauntless, the XSB2D was deemed to have better performance with the capability to carry a bigger bombload, and the Navy placed an order for 358 SB2D-1s (BuNos 04959/04971 and 09048/09384). By June, however, the Navy was shifting away from multi-seat dive bombers and focusing on a single-seat dive bomber designed to combine the combat capabilities of the SBD and SB2C with the strengths of a torpedo bomber, introducing the new BT (Bomber-Torpedo) category. Douglas immediately reworked the SB2D design into a single-seat dive/torpedo bomber to meet the Navy's new requirement, dispensing with the turrets and second crewmember, adding more fuel and armor, and equipping the wing racks with two torpedoes instead of one. This design was given the designation BTD-1 by the Navy and christened the Destroyer, and all orders for the SB2D-1 were converted to the BTD-1. The first flight of the BTD-1 occurred on March 5, 1944. but the Destroyer turned out to be heavier than the XSB2D-1 prototypes and had poorer performance, so Ed Heinemann asked the US Navy to cancel the BTD program. However, the Navy did not heed Heinemann's offer and still went ahead will full-scale production of the Destroyer, with deliveries to the Navy beginning in June 1944, and 28 BTD-1s were built before the end of World War II led to the remaining BTD-1s on order being canceled. Two BTD-1s (BuNos 04962 and 04964) were modified to house one 1,500 lb (6.7 kN) thrust Westinghouse 19B turbojet in the rear fuselage and designated XBTD-2, with the first flight occurring in May 1944. However, the turbojet did not deliver the expected improvement in performance because the Westinghouse turbojet underperformed at speeds of more than 200 miles per hour. Only the first XBTD-2, while second never flew. None of the BTDs saw combat in the Pacific theater of World War II, mainly being used for training purposes until 1947. In any case, by the time that the BTD went into production, Heinemann and his team were working on a dedicated dive/torpedo bomber entirely from scratch, the XBT2D-1, which would later become the AD/A-1 Skyraider.

Paradoxically, even though the SB2D/BTD Destroyer did not live up to its original goal of supplanting the SBD Dauntless in operational service due to changing requirements, at least it happened to be one of a few little-known US military aircraft of World War II that went into production (alongside the Consolidated B-32 Dominator and General Motors P-75 Eagle). That said, the mission philosophy for the BTD would eventually find its way into the Douglas AD/A-1 Skyraider and Martin BTM/AM Mauler, both of which were the last carrier-based tactical bombers to use piston power.

Competitors to the F-111 Aardvark from the LA area

The General Dynamics F-111 Aardvark fighter-bomber is one of the most controversial combat aircraft of the Cold War era to have entered service with the United States Air Force, ridiculed by several American politicians as the "Flying Edsel". Despite having a rocky start to its combat career during the Vietnam War in 1968 and congressional criticism, it eventually overcame several technical problems and became best known for conducting airstrikes on Libya on April  15, 1986 as part of Operation El Dorado Canyon, the US Air Force military operation to hold Libyan dictator Colonel Muammar al-Gaddafi accountable for his backing of international terrorism, including a bomb blast at a discotheque in West Berlin ten day earlier. The F-111 also was adapted for use as an electronic countermeasures (ECM) platform (EF-111 Raven) and an interim strategic bomber (FB-111), and it served with some USAF tactical combat units until the 1990s, when it was retired and replaced by the F-15E Eagle. In the past few years, I saw the F-111 in person for the first time, encountering a derelict  F-111D at the Yanks Air Museum and an FB-111 at the March Field Air Museum. Given that the F-111 was built by the Convair Fort Worth Division of General Dynamics in Forth Worth, Texas, I thought it would be worthwhile to discuss on this post rival designs from the Los Angeles area envisaged for the TFX (Tactical Fighter Experimental) contest that led to the F-111.

Fuselage hulk of a General Dynamics F-111D (serial number 68-0092) in the storage facility of the Yanks Air Museum in Chino, California. The General Dynamics F-111 design outcompeted the Boeing 818, Lockheed CL-590, and other designs in the TFX competition.

In 1958, the US Air Force began shopping for a new Mach 2 all-weather fighter that could also perform vertical takeoff and landing, but the SDR-17 (System Development Requirement) specification issued in February 1960 dropped the need for VTOL capability; the SDR-17 requirement was covered by US Air Force specification WS-324A. The SOR-183 requirement, which superseded SDR-17 in July, called for a two-seat Mach 2.5 fighter-bomber with all-weather capability and STOL performance to replace the Republic F-105 Thunderchief fighter-bomber, capable of carrying 15,000 lb (6,804 kg) of external stores and able to fly at an altitude of 60,000 feet (18,288 meters) with a low-level combat radius of 921 miles (1,482 km). However, plans to issue a Request for Proposals in October was put on hold, and by December the SOR-183 requirement was christened the TFX. After John F. Kennedy entered the White House, his Secretary of Defense, Robert S. McNamara, a staunch critic of manned strategic bombers and feverish advocate of intercontinental ballistic missiles, sought a new fighter-bomber to satisfy the parameters spelled out in the initial SOR-183/TFX requirement and the US Navy's VAX requirement for an A-4 Skyhawk replacement (which led to the Vought A-7 Corsair II), and in June 1961 he gave the go-ahead for the TFX program to proceed, culminating in September a revised SOR-183 requirement for a multi-role TFX. The proposed Air Force version was to have a maximum weight of 60,000 lb (27,216 kg) while the Navy all-weather fighter-bomber would weigh 55,000 lb (24,948 kg) with full avionics suite comprising an airborne missile control system to provide fire control, multiple target detection and tracking, and the multiple launch of long-range air-to-air missiles. Six designs for the TFX competition were submitted in late September by Boeing, General Dynamics plus Grumman, Republic plus Vought, Lockheed, McDonnell plus Douglas, and North American, all of them featuring swing wings.

Top: Drawing of the Lockheed CL-507-27, ancestor of the CL-590 project.
Bottom: Drawing (left) and full-scale mockup (right) of the CL-590 design for the TFX competition. 

In the run-up to the initiation of the TFX contest, in 1960 Lockheed had envisaged several design studies for a heavy STOL fighter-bomber under the designation CL-507. The first CL-507 iteration, the CL-507-1-1, had tapered fixed wings, a pair of jet engines in the rear fuselage, a length of 73 feet 2 in (22.30 meters), a wingspan of 43 feet 2 in (13.16 meters), and a wing area of 620 square feet (57.66 square meters). The next design study, called CL-507-4, had swing wings, two jet engines in individual pods, a length of 80 feet 6 in (24.54 meters), and a wingspan of 51 feet 4 in (15.65 meters), with the total wing area being 500 square feet (46.50 square meters) when the wings were in forward position or 458 square feet (42.59 square meters) when the wings were swept. The CL-507-5 was a delta wing proposal with canards as well as podded jet engines and a pair of 12,200 lb (54.2 kN) thrust lift fans in ahead of and behind the main landing gear, with a length of 94 feet 4 in (28.75 meters), a wingspan of 34 feet 8 in (10.57 meters), and a wing area of 600 square feet (55.80 square meters). The CL-507-27 was similar to the CL-507-4 in being a swing-wing design but differed in having the jet engines housed in the rear fuselage, and it was 77 feet 4 in (23.57 meters) long with a wing area of 500 square feet (46.50 square meters) and a wingspan of 60 feet (18.29 meters) when the wings were in forward position; the wings were swept at 20 degrees for subsonic speed and at 90 degrees in supersonic flight. After the TFX competition was announced by McNamara, in December 1961 Lockheed submitted a design for a multirole fighter-bomber under the designation CL-590, and eight versions were devised. The CL-590-1, the only variant for which company drawings are extant, had wings that spanned 60 feet 6  in (18.44 meters) in forward position and 29 feet 8 in (9.04 meters) when fully swept, and it would have attained Mach 1.2 at sea level and Mach 2+ at altitude. The Air Force and Navy versions of the CL-590-1 slightly differed in length and gross weight, with the Air Force variant being 67 feet 5 in (20.54 meters) long and weighing 59,209 lb (26,857 kg) fully loaded, whereas the Navy variant was 65 feet 8 in (20.02 meters) long with a gross weight of 55,000 lb (24,948 kg). Power was supplied by two General Electric MF295A turbofans mounted alongside the sides of the rear fuselage, and the crew sat in a tandem fashion, and armament provisions were made for different types of air-to-surface missiles and bombs. The pylons for mounting ASMs or bombs would have had to rotate in order to remain parallel with the fuselage of the CL-590-1 as the wings were swept forward or back. 

Artist's rendering of the US Air Force version of the one of North American's designs for the TFX competition 

Very little technical information exists for the TFX design studies from North American Aviation, apart from artist's conceptions by North American, which were recently uncovered in the Boeing archives. However, North American apparently investigated three TFX configurations, all similar to each other in having a long, sleek fuselage, a raised cockpit canopy and forward fuselage, and turbofans mounted side-by-side in the rear fuselage. The primary design (which includes the only available images of the Air Force and Navy versions) featured side-fuselage air intakes below the fixed inner wings and twin vertical stabilizers, and second design was similar to the first but with the air intakes positioned further forward to the fixed inner wing's leading edge. The third proposal had a single vertical stabilizer along with a distinct jet pipe arrangement and forward-swept intakes for the turbofans ahead of the wing roots (Buttler 2021, pp. 229-232). Mention must be made of the fact that the El Segundo Division of Douglas was enlisted as a team member by McDonnell for that company's Model 156 submission, which was investigated in both side-by-side and tandem seating arrangements for the crew. The Air Force and Navy versions of the Model 156 with side-by-side seating were 80 feet 8 in (24.59 meters) and 65 feet 6 in (19.96 meters) in length respectively, while the tandem seating proposals were slightly smaller, with the Air Force design 73 feet (22.25 meters) long and the Navy design measuring 62 feet (18.90 meters) long. Armament provisions for the Air Force version included GAM-83 Bullpup and GAR-8/AIM-9 Sidewinder missiles as well as demolition and fire bombs, while the Navy variant would have been armed with long-range air-to-air missiles or AIM-7C Sparrow III AAMs, Bullpups, and demolition and napalm bombs (Butler 2010, p. 138).   

On November 24, 1962, the Defense Department declared the General Dynamics/Grumman design the winner of the TFX competition, largely because the Air Force and Navy variants of the GD/Grumman submission had greater commonality compared to other contending designs, and on December 21, the aircraft was designated F-111, with an order placed for 18 F-111As (to be built by General Dynamics) and five F-111Bs (to be built by Grumman). The F-111A took to the skies on December 21, 1964, and in spite of engineering problems early in the F-111's service life, over 550 F-111s were built, serving not only in the Vietnam War but also in Operation El Dorado Canyon and Operation Desert Storm in early 1991. 

References:

Buttler, T., 2010. American Secret Projects: Bombers, Attack, and Anti-Submarine Aircraft 1945 to 1974. Hinckley, UK: Midland Publishing.

Buttler, T., 2021. American Secret Projects 4: Bombers, Attack, and Anti-Submarine Aircraft 1945 to 1974. Manchester, UK: Crécy Publishing.