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.