History
The development of the first P-39 was initiated by the Circular Proposal X-609. It was issued by Project Officer for Fighters, Lieutenant Benjamin S. Kelsey of the US Army Air Corps, and fighter tactics instructor, Captain Gordon P. Saville of the Air Corps Tactical School, in 1937. It called for the request for a new high-altitude interceptor aircraft with a single engine. Specifications included at least a thousand pounds of heavy armament with a 37-mm cannon, a liquid-cooled Allison engine with a General Electric two-stage turbo-supercharger, tricycle landing gear, a level airspeed of at least 360 mph at altitude, and a climb to 20,000 feet within six minutes. With these specifications, the aircraft must be able to conduct the tactical mission of interception attack on hostile aircraft at high altitudes. The requirements presented the most difficult challenge the USAAC had set for the designers.
The aerospace manufacturer Bell Aircraft of New York presented its design, “XP-39.” The XP-39 first flew on April 6, 1938, at Wright Field, Ohio, achieving 390 mph at 20,000 feet in just five minutes. The XP-39 had a turbo-supercharger to improve its high-altitude performance, as originally required by Kelsey and Saville. The turbo was cooled with a scoop on the left side of the fuselage. The National Advisory Committee for Aeronautics evaluated the aircraft and listed recommendations to allow it to reach the requirements, such as increasing its speed, reducing drag, and improving the V-1710 with only a single-stage, single-speed supercharger, and concluded that the top speed of 429 mph could be achieved this way.
In August 1939, Bell attended the meeting with the USAAC and NACA and proposed that the production P-39 aircraft was to be configured without the turbocharger. The sources have estimated that the reconfiguration was motivated by the lack of an additional Bell production program to support the turbo, the disappointing results of unoptimized turbo installation discovered in the wind tunnel tests, and USAAC’s doctrine that the Atlantic and Pacific Oceans made the USA virtually immune from high-altitude attack by enemy bombers.
The Army placed an initial order of 12 YP-39s, configured with single-stage, single-speed superchargers, and one YP-39A in April 1939. They were tested and evaluated for entry to service, and the new recommendations were noted, with the need for deletion of the external radiator and other details changes. An XP-39B was reconstructed from the XP-39 to follow this new configuration. Following the successful improvement in the test flight of XP-39B on November 25, 1939, 13 YP-39s were reconfigured to this standard.
The XP-39B suffered severe, rapid degradation in high-altitude performance over 15,000 feet due to the removal of the turbocharger. Its maximum speed fell from 390 mph at 20,000 feet to 375 mph at 15,000 feet, and it took 7.5 minutes to reach 20,000 feet instead of five minutes. In return, the XP-39B’s low-altitude maneuverability was increased with the reduced wing span.
The first YP-39 was flown on September 13, 1940, with the 1090 hp V-1710-37. The later YP-39s were fitted and armed with a 37-mm cannon with 15 rounds, a pair of .50-cal machine guns with 200 rounds per gun, and a pair of .30-cal machine guns with 500 rounds per gun. All guns were installed in the nose. The YP-39s lacked armor and self-sealing fuel tanks and were 2,000 lb lighter than the production P-39. The flight tests showed the performance of the YP-39 dropped to a maximum speed of 368 mph at 15,000 feet. It took them 7.3 minutes to reach 20,000 feet with a service ceiling of 33,300 feet. The evaluation and testing of YP-39s were completed on December 16, 1940, as the Army had placed an order for 80 P-39Cs (formerly P-45) on August 10, 1939.
The first flight by a P-39C was made in January 1941, and only 20 P-39Cs were built before switching to building 60 P-39Ds. Most P-39Cs were assigned to the 40th Pursuit of the 31st Pursuit Group at Selfridge Field, Michigan, where they were used as service test aircraft. They were delivered in the standard Olive Drab over a Neutral Gray paint scheme, and they had national insignias on both fuselage sides and the upper left and lower right wing positions. They did not have the red, white, and blue stripes on the vertical tail and rudder.
Consequently, the 20 P-39Cs were declared not combat-ready for operational use due to the aerial combat reports in Europe, as the armor and self-sealing tanks were needed for the production fighters. No P-39Cs were used in combat, and the remaining 60 P-39Cs were reconfigured to P-39Ds with armor, self-sealing tanks, and enhanced armament.
Only the three P-39Cs were ever delivered as British aircraft to the United Kingdom. They arrived at RAF Colerne on July 3, 1941, and were subjected to further evaluations.
