P-39

P-39 Airacobra
P-39Q 42-1947, "Saga Boy II" of Lt. Col. Edwin S. Chickering, CO 357th Fighter Group, July 1943
Role	Fighter
National origin	United States
Manufacturer	Bell Aircraft
First flight	6 April 1938
Introduction	1941
Status	Retired
Primary users	United States Army Air Forces Soviet Air Force Royal Air Force
Produced	1940 – May 1944
Number built	9,588
Unit cost	50,666 USD in 1944
Variants	Bell XFL Airabonita Bell P-63 Kingcobra

The Bell P-39 Airacobra was one of the principal American fighter aircraft in service when the United States entered World War II. The P-39 was used by the Soviet Air Force, which scored the highest number of individual kills attributed to any U.S. fighter type in the Eastern European theatre. Other major users of the type included the Free French, the Royal Air Force, the United States Army Air Forces, and the Italian Co-Belligerent Air Force.

Designed by Bell Aircraft, it had an innovative layout, with the engine installed in the center fuselage, behind the pilot, and driving a tractor propeller via a long shaft. It was also the first fighter fitted with a tricycle undercarriage. Although its mid-engine placement was innovative, the P-39 design was handicapped by the absence of an efficient turbo-supercharger, preventing it from performing high-altitude work. As such it was rejected by the RAF for use over western Europe but adopted by the USSR where most air combat took place at medium and lower altitudes.

Together with the derivative P-63 Kingcobra, the P-39 was one of the most successful fixed-wing aircraft manufactured by Bell.

In February 1937, Lieutenant Benjamin S. Kelsey, Project Officer for Fighters at the United States Army Air Corps (USAAC), and Captain Gordon P. Saville, fighter tactics instructor at the Air Corps Tactical School, issued a specification for a new fighter via Circular Proposal X-609. It was a request for a single-engine high-altitude "interceptor" having "the tactical mission of interception and attack of hostile aircraft at high altitude". Despite being called an interceptor, the proposed aircraft's role was simply an extension of the traditional pursuit (fighter) role, using a heavier and more powerful aircraft at higher altitude. Specifications called for at least 1,000 lb of heavy armament including a cannon, a liquid-cooled Allison engine with a General Electric turbo-supercharger, tricycle landing gear, a level airspeed of at least 360 mph (580 km/h) at altitude, and a climb to 20,000 ft (6,100 m) within 6 minutes. This was the most demanding set of fighter specifications USAAC had presented to that date. Although Bell's limited fighter design work had previously resulted in the unusual Bell YFM-1 Airacuda, the Model 12 proposal adopted an equally original configuration with an Allison V-12 engine mounted in the middle of the fuselage, just behind the cockpit, and a propeller driven by a shaft passing beneath the pilot's feet under the cockpit floor.