Southern California's lost long-range interceptors, part 2: Lockheed YF-12

The cancellation of the F-108 Rapier in September 1959 was a setback to efforts by the US Air Force to field an advanced long-range interceptor to take on advanced strategic bombers being developed by the USSR, including the Myasishchev M-50 and Tupolev Tu-22. However, the quest by the aircraft industry of southern California to provide the Air Defense Command with a Mach 3 interceptor did not totally die out with the F-108's termination. Not too long after the F-108 program was axed, a revolutionary aircraft of similar speed to the F-108 would come to form the basis for another US effort to acquire a Mach 3 long-range interceptor, this time from Lockheed.

Desktop model of the YF-12 at the Planes of Fame Museum, Chino (photographed by me in 2018)

In 1960, while undertaking development of the A-12 (Archangel-12) spy plane, Lockheed proposed a long-range interceptor variant of the A-12 for the US Air Force under the internal designation AF-12. The AF-12 design was to have provisions for a second crewmember (radar operator), missile bays for folding fin versions of the GAR-9 (later AIM-47) long-range air-to-air missile, and a Hughes ASG-18 fire control search-and-track radar. Pleased with the AF-12 proposal, in October 1960 the US Air Force signed a contract whereby the seventh through ninth A-12s on order (serial numbers 60-6934/6936) would be completed as AF-12s. The CIA assigned the codename KEDLOCK to the AF-12 program, and a mock-up was inspected by Air Force officials in May 1961, after which the AF-12 design was tweaked the following month to include three ventral fins (two below the engine nacelles and a third larger fin below the extreme rear of the fuselage). To accommodate the Hughes ASG-18 fire-control radar, the chines were cut back insofar that they extended to the front of the cockpit, and the nose was now made of plastic composite material that was transparent to radar and resistant to thermal loads expected from Mach 3+ flight. The AF-12 had a length of 101 feet 8 in (30.97 meters), a wingspan of 55 feet 7 in (16.95 meters), a height of 18 feet 6 in (5.64 meters), a wing area of 1,795 square feet (167 square meters), an empty weight of 60,730 lb (27,604 kg), a gross weight of 124,000 lb (56,200 kg), and a maximum takeoff weight of 140,000 lb (63,504 kg).

Select photos of the Lockheed YF-12A in flight 

Construction of the first AF-12 began in August 1962 at the Lockheed factory in Burbank, California, where other A-12s were being manufactured. After the Defense Department introduced the Tri-Service aircraft designation system on September 18, 1962, the AF-12 was officially designated YF-12 as a matter of convenience given its A-12 heritage. The first YF-12A was completed in the first half of 1963 and transported to Area 51 in Groom Lake, Nevada, where it made it first flight on August 7; the second YF-12A aircraft flew for the first time on November 23, 1963, and the third prototype followed suit on March 13, 1964. The YF-12A's existence was publicly disclosed on February 29, 1964 when President Lyndon B. Johnson referred to it as "A-11" at his first nationally televised news conference, and the aircraft was publicly unveiled at Edwards Air Force Base on September 30. For security reasons, LBJ may have chosen to referred to the YF-12A as "A-11" because the Soviets had breached Lockheed and CIA security, even though the A-11 was a fundamentally different design than the YF-12A (Lowther 2021, pp. 99-100). In the meantime, the AIM-47 Falcon was ejected from a YF-12A for the first time on April 16, 1964, and on March 18, 1965 the YF-12A scored its first kill when an AIM-47 hit an aerial  target 36.2 miles (58.26 km) away. On May 1, 1965, the first and third YF-12As set speed and altitude records of 2,070 miles per hour (3,331.505 km/h) and 80,258 feet (24,462.60 meters) respectively, and four months later, on September 28, the first YF-12A fired an AIM-47 at a speech of Mach 3.2 at an altitude of 75,000 feet (22,860 meters), intercepting a JQB-47E target drone 32.2 miles (51.8 km) away at 500 feet (150 meters). In all, 13 missile firings were conducted from the YF-12A up to 1966.

Forward fuselage mock-up of the Lockheed F-12B, the proposed production F-12

The USAF's Air Defense Command was quite impressed by test flights of the YF-12A, and May 14, 1965 it placed a production order for 93 F-12Bs. The F-12B was similar to the YF-12A but differed in utilizing the nose chines of the A-12 and SR-71 but also eliminating the central large stabilizing fin below the rear fuselage, not to mention that the underside of the forward fuselage was flattened and the tail of the rear fuselage extended as in the SR-71. Instead of using ejectors to toss the AIM-47s from the internal weapons bays, the F-12B would have used deployable trapeze launch rails that could deploy the missiles in a substantially nose-down altitude. Plans called for spreading out the F-12B fleet to different Air Force bases, with three squadrons of 16 F-12Bs each sent to bases in the northeast US, and three squadrons of 16 aircraft deployed on the west coast of the US. However, US Secretary of Defense Robert McNamara did not support development of the F-12B, and due to Vietnam War cost and a lower priority placed by updated intelligence analysts upon defense of the continental US, the F-12B program was cancelled in January 1968. Not too long before the F-12B's cancellation, Lockheed had also looked into a fighter-bomber version of the F-12, unofficially called FB-12 by the company, which resembled the SR-71 and would carried either four AGM-69 nuclear-armed land attack missiles, two AGM-69s and two AIM-7 Sparrow air-to-air missiles, four AIM-7s, three AIM-7s and one M61 Vulcan machine gun, four AIM-47s, or three AIM-47s and one M61. However, the FB-12 went no further than the drawing board because the FB-111 and F-15 would soon be developed to fulfill the FB-12's intended air-to-air and air-to-ground roles. In the meantime, the first YF-12A (serial number 60-6934) suffered a landing mishap that caused the forward fuselage to be damaged beyond repair, leaving the remainder of the first YF-12A to be mated with a static test SR-71 airframe to create the SR-71C two-seat trainer. The second and third YF-12A later were transferred to NASA in 1969 for scientific missions ranging from sonic boom tests to high-altitude air sampling; serial number 60-6936 was lost in a crash near Edwards Air Force Base on June 24, 1971 after an in-flight fire, but both pilots ejected safely, while the sole remaining YF-12A continued serving NASA until November 17, 1979, when it was flown to the US Air Force Museum at Wright-Patterson Air Force Base in Dayton, Ohio. (NASA acquired SR-71 serial number 61-7951 in 1971 for aerodynamic research purposes after the crash of the third YF-12A and assigned it the bogus designation YF-12C and the fair serial number 60-6937 due to the secrecy surrounding the SR-71, operating this aircraft until December 1978, but that is another story.)

Although the F-108 Rapier was canceled without ever reaching the hardware phase and the F-12 almost made it into production when the Defense Department canceled F-12 production plans after deciding that defense of the continental US wasn't too important, some legacies of the F-108 and F-12 programs would find their way into future jet fighters. For example, the AIM-47 Falcon designed to be carried by the F-108 and F-12 would form the basis of the AIM-54 Phoenix, the chief armament of the US Navy's F-14 Tomcat, while the ASG-18 radar was later developed into the more advanced AWG-9 and APG-71 all-weather radars that equipped the F-14. Today, the F-15 Eagle and F-22 Raptor utilize the long-range air-to-air combat role envisaged for both the F-108 and F-12 in tandem with the air superiority role for which they are primarily designed.

References:

Jenkins, D.R., and Landis, T.R., 2008. Experimental & Prototype U.S. Air Force Jet Fighters. North Branch, MN: Specialty Press.

Landis, T.R., and Jenkins, D.R., 2005. Lockheed Blackbirds (Warbird Tech Series, Volume 10), Revised edition. Minneapolis, MN: Specialty Press. 

Lowther, S., 2021. Lockheed SR-71 Blackbird - Origins and Evolution. Horncastle, UK: Tempest Books.

Southern California's lost long-range interceptors, part 1: North American F-108 Rapier

For many US aircraft manufacturers, the 1950s would see quantum leaps in combat aircraft technology when it came to supersonic speed, punctuated by the USAF's introduction of its first supersonic jet fighter, the North American F-100 Super Sabre, but also the Douglas D-558-2 Skyrocket becoming the first plane to reach Mach 2 in November 1953. The Convair F-102 Delta Dagger and F-106 Delta Dart, first flown in 1953 and 1956 respectively, constituted quantum leaps in the development of US supersonic fighter planes because they gave America the first-ever purpose-built supersonic interceptors to take down Soviet long-range bombers intruding into North American airspace. With development of the North American XB-70 Valkyrie Mach 3 supersonic bomber underway, however, the US Air Force began shopping for an all-new long-range interceptor capable of traveling at Mach 3 and countering the USSR's upcoming supersonic bombers like the Myasishchev M-50, Tsybin RSR, and Tupolev Tu-22. In response, the aircraft industry in southern California unveiled a spree of Mach 3 long-range interceptor designs, some conceived from scratch and others derived from existing aircraft designs. Due to the prolonged quest by the USAF for a Mach 3 interceptor, I am writing a two-post series on Mach 3 interceptor design in the Los Angeles basin; the first post will focus on the North American F-108 Rapier.

Top: Northrop N-144 interceptor design submission for the 1954 LRI competition
Bottom left: Desktop model of the North American submission to the 1954 LRI competition
Bottom right: An artist's conception of the North American NA-236, the winner of the LRI-X competition and the final step towards the F-108 Rapier  

In late April 1954, the USAF announced the Long-Range Interceptor (LRI) program for an advanced long-range interceptor with an operating altitude of 60,000 feet (18,000 meters) at Mach 1.7 (1,122 mph (1,806 km/h) with a maximum range of 1,151 miles (1,852 km), and the ability to detect enemy targets over a distance of 115 miles (185 km). The parameters for the LRI requirement were covered under the designation WS-202A, and ten companies undertook design studies for the LRI requirement, of which seven submitted bids. Northrop submitted a revised design for its earlier N-126 'Delta Scorpion' design for the WS-202A specification with a long, slim fuselage, turbojets below the wings, and low-mounted horizontal stabilizers, and it also worked out two additional designs, the N-144 (a scaled-up version of the N-126) and the lightweight N-149. North American's WS-202 submission had twin upper and lower vertical stabilizers, a cropped delta wing, a long, slim fuselage, and delta wing canards, and Lockheed's submission, the CL-288, looked like an F-104 Starfighter with mid-wing mounted turbojets. None of the seven designs offered for the LRI competition fully met the performance requirements in the LRI specification, but the N-144 was judged by Wright Field to come closest to meeting the parameters of the LRI requirement. As the submissions for the LRI requirement did not meet the required operational altitude, the evaluators at Wright Field asked for a relaxation of the original requirements, and on July 20, 1955, the US Air Force initiated the LRI-X (Long-Range Interceptor - Experimental) program, which like the earlier LRI program stipulated that a new-generation long-range interceptor should fly at 60,000 feet (18,000 meters) at a speed of Mach 1.7 (1,122 mph (1,806 km/h), but called for the new interceptor to utilize an integrated fire-control system to allow for interception of a bomber over a range of 69 miles (111 km), with the ability to make three kills. The GOR-114 requirement was initiated on October 6 to cover the LRI-X operational parameters, and by October 11, the USAF awarded study contracts to three companies (Lockheed, Northrop, and North American). The Lockheed CL-320 was similar to the CL-288 but was much larger and heavier, and it was powered by four General Electric J79 turbojets housed in two mid-wing nacelles, along with outrigger landing gear below the nacelles. North American proposed the NA-236, which resembled the 1954 North American interceptor proposal in having delta canards but differed in utilizing a delta wing, the canards atop the forward fuselage, and two side-by-side General Electric J93 turbojets. A large central vertical stabilizer was situated between the turbojets, and two smaller vertical stabilizers were mounted in mid-wing position along the wing's trailing edge. Northrop's LRI-X design, the N-167, had four General Electric J79 turbojets housed inside the fuselage, fed by air flowing through intakes in the wing roots, and two designs were investigated, the baseline N-167 with a tail empennage similar to that of the F-104 and wings with pronounced anhedral, and the N-167A design of April 1956 with the horizontal stabilizer on the rear side of the fuselage and of the same span as the wings. By January 1956, the NA-236 was declared the winner of the LRI-X contest, but budgetary constraints forced the Pentagon to cancel the LRI-X program on May 9. At the behest of the Air Defense Command, however, the Pentagon revived the long-range interceptor program on April 11, 1957, and in June of that year North American won a contract for the aircraft, which eventually was designated F-108.The Air Defense Command at the time was hoping to procure 480 F-108s.

Top: Model of the North American F-108 Rapier at the Lyon Air Museum, photographed by me in November 2021.
Bottom: Full-scale mockup of the F-108 at the North American plant in Inglewood, California, January 1959.

The initial XF-108 design (company designation NA-257) conceived in early May 1958 was similar to the NA-236 but had the trailing edge flaps split into two downward curving surfaces. It measured 84 feet 11 in (25.9 meters) long with a wingspan of 52 feet 11 in (16.1 meters), a wing area of 1,400 square feet (130.2 square meters), and a gross weight of 99,400 lb (45,088 kg), and it was powered by two General Electric J93s. Beginning in September, however, the F-108 design was revised whereby the canards were removed and larger underwing vertical stabilizers were provided to offer better handling and directional stability at low speeds and high angles of attack. Three months later, the wing design for the F-108 was changed to a cranked arrow shape with substantial tip extensions offering improved stability at high lift conditions, and large folding ventral stabilizers were fitted to the corners of the lower fuselage to address concerns about directional stability. In this form, the overall F-108 design was to be 89 feet 2.5 in (27.2 meters), with a wingspan of 57 feet 5 in (17.5 meters), a height of 22 feet 1 in (6.7 meters), a wing area of 1,865 square feet (173.4 square meters), an empty weight of 50,907 lb (23,098 kg) and a gross weight of 102,234 lb (46,373 kg). The F-108 was armed with three GAR-9 (later AIM-47) Falcon long-range air-to-air missiles, which had a range of 115 miles (185 km/h), and the top speed was to be Mach 3, the same speed as the company's own XB-70 Valkyrie supersonic strategic bomber. A full-scale mock-up of the F-108 was completed and inspected by US Air Force officials in January 1959, and the F-108 was officially christened Rapier in May. By this time, the first flight of the F-108 was scheduled for March 1961 with service entry planned for mid-1963, and the USAF order for 30 service test F-108s was reduced from 30 to 21 aircraft. However, the Pentagon was facing a tight budget stemming from the development of the Atlas and Titan ICBMs, the XB-70 Valkyrie supersonic bomber, the Polaris SLBM, and the Navy's fielding of its first generation of ballistic missile submarines, and given the rising cost of the F-108 program, on September 23, 1959, the USAF announced the cancellation of the Rapier. Several months before the F-108's cancellation, there was a proposal for a Mach 3 target drone for the testing the F-108's weapons systems, and in July 1959 the designation XQ-11 was requested for this vehicle, but the USAF Headquarters turned down the request, stating that a specific designation for the proposed F-108 target was unnecessary at an early stage of the F-108 program, and like the F-108, the XQ-11 did not proceed to the hardware phase.

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.

Jenkins, D.R., and Landis, T.R., 2008. Experimental & Prototype U.S. Air Force Jet Fighters. North Branch, MN: Specialty Press.

Experimental pusher fighters from the Los Angeles basin: Vultee XP-54 and Northrop XP-56

The annals of US fighter development in World War II before and after the Japanese attack on Pearl Harbor in December 1941 were defined by a proliferation of advanced designs for piston-engine fighter planes to either supplant or augment existing fighters in service with the US Army Air Force, namely the P-40 Warhawk, P-47 Thunderbolt, and P-51 Mustang. However, several fighter plane designs for the USAAF stood out relative to other designs in that utilizing radical design philosophies, including the pusher-engine layout, tailless flying wing layout, and swept wings. Among the radical USAAF piston-engine fighter projects conceived in the crucible of World War II, two designs were created on the drawing boards of the aircraft industry in the Los Angeles basin of southern California, the Vultee XP-54 and Northrop XP-56 pusher-engine fighters. 

Models of the Vultee XP-54, Curtiss XP-55, and Northrop XP-56, the winners of the R-40C competition. Photo taken by me at the Lyon Air Museum in November 2021. 

On November 27, 1939, the US Army Air Corps initiated Pursuit Specification XC-622 for a single-seat, single-engine fighter able to climb to 20,000 feet (6,096 meters) in 7 minutes and reach 425 miles per hour at altitudes of 15,000-20,000 feet (4,572-6,096 meters). The USAAC considered the 1,800 hp Pratt & Whitney H-3130 liquid-cooled H-block piston engine as having sufficient power to allow the fighter plane covered by XC-622 to meet the stated requirements. By February 20, 1940, the USAAC announced the Request for Data R-40C, which covered the requirements laid out in XC-622, and a total of six companies (Bell, Curtiss, McDonnell, Northrop, Republic, and Vultee) submitted bids; technical details of the proposals for the R-40C competition are discussed in Balzer (2008). Vultee's submission, the Model 70, was a twin-boom fighter with a length of 37 feet 6 inches (11.43 meters), a wingspan of 40 feet (12.2 meters), one Pratt & Whitney H-2600 H-block piston engine in pusher configuration, and armament comprising two .30-caliber and two .50-caliber machine guns plus two 20 mm cannons in the nose; three versions of the Model 70 were investigated, of which Versions 1 and 3 both had three-blade counter-rotating propellers but differed in gross weight (9.000 lb for Version 1, 9,055 lb for Version 3), while Version 2 was to use a single four-bladed propeller and weigh 8,788 lb (3,986 kg) when fully loaded (Buttler and Griffith 2015, p. 26). Northrop's proposal, designated N-2, was a tailless aircraft with drooping wingtips similar to those of the N-1M experimental flying wing and one ventral vertical fin , and it also featured a piston engine arranged in pusher configuration. Five variants were envisaged, with various powerplants investigated including the Allison V-1710, Pratt & Whitney R-1830 Twin Wasp, Pratt & Whitney R-2800 Double Wasp, and Pratt & Whitney H-2600. The first three N-2 designs (N-2, N-2A, and N-2B) were armed with two .50-caliber machine guns and two 20 mm cannons in the nose, while the N-2C and N-2D had two .30-caliber machine guns substituted for the .50-caliber guns (Buttler and Griffith 2015, p. 24). In late May 1940, the Vultee Model 70 and Northrop N-2B were selected by the USAAC for full-scale development and designated XP-54 and XP-56 respectively. (The Curtiss P-249C, the most unorthodox of the Curtiss submissions to R-40C, was designated XP-55 after being selected for full-scale development by the Army Air Corps over the orthodox CP-40 and P-248, but this aircraft) On September 26, one XP-56 prototype (serial number 41-786) was ordered, and a few months later, on January 8, 1941, a contract was signed for one XP-54 prototype with the serial number 41-1210). The US Army Air Force later ordered a second XP-54 prototype (serial number 42-108994) on November 28, 1941 (contract signed March 17, 1942), and a contract was issued for a second XP-56 prototype (serial number 42-38353) on February 13, 1942. The XP-54 was nicknamed "Swoose Goose" after a song about Alexander who was half-swan and half-goose, dubbed Alexander the Swoose. The XP-56 was nicknamed the Black Bullet, Silver Bullet, or Dumbo, but none of these names were ever officially applied to this aircraft.

Top: First XP-54 prototype (serial number 41-1210) at Muroc Army Air Field, early 1943.
Bottom: Second XP-54 (serial number 42-108994) in flight over the San Bernandino Mountains, early 1944.  

Even before the XP-54 prototype contract award, in December 1940 the powerplant for the XP-54 changed to one Lycoming XH-2470 H-block flat piston engine after Pratt & Whitney canceled the H-2600 program in October due to disappointing results of test runs of the H-2600. A full-scale mock-up of the XP-54 was inspected in May 1941, by which time the US Army Air Corps now wanted a high-altitude fighter with a pressurized cabin rather than a medium-altitude, high-speed fighter. In response, Vultee reworked the XP-54 design to incorporate a pressurized cockpit, a longer fuselage, larger wings, greater armor protection, and a turbo-supercharged engine, and the revised design of the XP-54, called the Model 84, was 53 feet 10 in (16.41 meters) long, with a wingspan of 54 feet 9 in (16.69 meters), a wing area of 456 square feet (42.4 square meters), an empty weight of 15,262 lb (6,923 kg), a gross weight of 18,233 lb (8,270 kg), and two 37 mm cannons and two .50-caliber machine guns in the nose. In September 1941 Vultee discussed with the US Army Air Force a proposed version of the XP-54 with one Wright R-2160 Tornado radial piston engine, and the designation XP-68 was assigned. The option was made to complete the second XP-54 prototype with the Tornado engine, but expected lengthy development of the Tornado engine led to the XP-68 project being cancelled on November 22, 1941. Meanwhile, the first XP-54 prototype was completed in late 1942 and made its first flight on January 15, 1943, with test pilot Frank Davis at the controls. Over 100 flights were carried out until October 28, when the first XP-54 was flown to Wright Field, Ohio, for service tests. Despite showing good handling characteristics, the XP-54 reached a top speed of only 380 mph (611 km/h), far below the guaranteed  top speed jotted out by the USAAF, and the Lycoming engine was hamstrung by serious technical woes, so the US Army Air Force decided not to order the XP-54 into production. The possibility of fitting the first XP-54 prototype with an Allison V-3420 liquid-cooled V-cylinder piston engine was considered, as was a proposal to fit the XP-54 with a turbojet, but both schemes were rejected as cost-prohibitive. The second XP-54 made its first flight on May 24, 1944, and ten flights were carried out until April 2, 1945, but the engine-turbosupercharger combination was pronounced unreliable and returned to Vultee to be replaced by another engine. (Many sources say that the second XP-54 flew only once, but this is not borne out by existing flight logs.) Oddly, photos of the second XP-54 in flight show the aircraft with the serial number 41-1211 on the vertical stabilizer, but this serial was actually allocated to a Vultee BT-13 Valiant trainer, and the second XP-54 most likely had 41-1211 applied to the vertical stabilizer in error, because it was ordered several months after the first XP-54, and there is a photo of one North American RF-100A Super Sabre erroneously marked with the serial number 53-2600 (which was actually used for a Northrop F-89 Scorpion). Neither of the XP-54s survive today, the first aircraft having been tested to destruction during static tests at Wright Field and the second XP-54 being scrapped after its last flight. 

Left: First Northrop XP-56 (serial number 41-786) during taxi tests in early September 1943
Right: Second Northrop XP-56 (serial number 42-38353) in flight

The other winner of the R-40C competition developed in the Los Angeles basin, the XP-56, was 27 feet 6 inches (8.38 meters) long and 11 feet (3.35 meters), with a wingspan of 42 feet 6 inches (12.96 m), a wing area of 306 square feet (28.44 square meters), an empty weight of 8,700 lb (3,955 kg), a gross weight of 11,350 lb (5,159 kg), and maximum takeoff weight of 12,145 lb (5,520 kg). The anticipated speed of the XP-56 was to be 465 miles per hour (749 km/h), and armament comprised four .50-caliber machine guns and two 20 mm cannons. After being told that Pratt & Whitney had canceled the H-2600 program, Northrop had the powerplant for the XP-56 design changed to one Pratt & Whitney R-2800 Double Wasp radial piston engine driving counter-rotating propellers. The XP-56 had to be constructed from magnesium alloy due to wartime shortages of aluminum, so Northrop engineer Vladimir Pavlecka utilized the new heliarc welding technique (company designation N-13) to manufacture magnesium in large sections. The first XP-56 was completed in March 1943 and underwent engine tests at before being shipped to Muroc Army Air Field in April for ground tests, but problems arose during taxi trials, including faulty wheel brakes, necessitating the installation of manual hydraulic brakes. The first flight flight of the XP-56 took place on September 6, 1943, piloted by John Myers, but yaw problems and nose heaviness were noticed during testing, so a dorsal vertical stabilizer was fitted in order to cure stability problems. A few additional flights were made at Muroc, but on October 8, the aircraft suffered tire blowout during a high-speed taxi run, causing the XP-56 to flip over and become wrecked. Myers himself survived with minor injuries, largely thanks to him wearing a polo player's helmet. After the crash of the first XP-56, Northrop decided to replace the cotton fabric tires of the first prototype with virgin rubber tires on the second XP-56 prototype, which was completed in January 1944. The second XP-56 first flew on March 23, with Northrop test pilot Harry Crosby at the controls, and a total of ten flights were carried out. Although the nose heaviness disappeared when the landing gear was retracted, stability still remained an issue, and the second XP-56 attained 320 miles per hour (515 km/h) instead of the projected 465 mph (749 km/h). On May 30, 1944, NACA was asked to use its wind tunnel at Moffett Field, California, to investigate the causes of the XP-56's low performance, but after complaints during the tenth flight of the second XP-56 about extreme tail heaviness on the ground, low power, and excessive fuel consumption, the USAAF decided not to conduct any more flights tests, and the XP-56 program was abandoned in late 1944. The second XP-56 prototype, miraculously, escaped the breaker's torch; on December 20, 1946, US Army shipped it to Freeman Field, Indiana, to be put into long-term storage. The second XP-56 became part of the National Air and Space Museum's collection in 1950 –1951 when the Smithsonian moved it to the Paul Garber Restoration Facility in Suitland, Maryland, where it sits today.

Never-realized derivatives of the XP-54: the Vultee Model 78 (left) and carrier-based Model 79 (right)

In an interesting footnote, in early January 1941 Vultee proposed a version of the XP-54 for the export market, the Model 78, which measured 48 feet (14.63 meters) in length and had a wingspan of 46 feet (14.02 meters), a gross weight of 8,500 lb (3,855 kg), and six .50-caliber machine guns in the nose, with power provided by one Allison V-1710 liquid-cooled V-cylinder piston engine (Balzer 2008, pp. 60-61). The same month, a carrier-based derivative of the XP-54, the Model 79, was envisaged for the Navy's SD-112-18 requirement, and two variants were devised, the 79A and 79C, which slightly differed in length, wingspan, and gross weight, not to mention that the Model 79A used counter-rotating propellers whereas the Model 79C had a single propeller (Buttler and Griffith 2015, p. 166). The Model 78 did not progress beyond the drawing board, and the Model 79 design lost out to the Grumman Model 51 (which became the F7F Tigercat) in the SD-112-18 contest. Northrop, for its part, looked at a derivative of the XP-56 powered by a turbojet (probably a Westinghouse J30 or General Electric J31), recognizing the onset of the jet age, but this proposal never materialized because Northrop focused its attention on P-61 production and development XB-35 flying wing bomber and XP-79B flying wing jet fighter (Chong 2016, p. 15).

References:

Balzer, G., 2008. American Secret Pusher Fighters of World War II: XP-54, XP-55, and XP-56. North Branch, MN: Specialty Press.

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

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