A very radical aircraft, the Bell P-39 Airacobra was built around a massive hub-firing 37 mm cannon which required the engine to be situated quite unconventionally behind the cockpit and the propeller driven by an extension shaft which passed between the pilot's legs (it also housed a tricycle landing gear). Along with the P-40, the Airacobra was the USAAF's main fighter for the first months of the war but was hard pressed against the superior Japanese Zero in the Pacific and German fighters in Europe. Still, despite the fact it was a quite unforgiving aircraft to fly almost half of all P-39s (4,773 in all) ended in Soviet hands as part of Lend-Lease aid where they found a more appropriate low-altitude environment more suitable to its performance characteristics. In USAAF service they fought in the Aleutians, South Pacific and the Mediterranean but were also used by the RAF as well as the Free French and Italian Co-Belligerent air forces. Meanwhile, a slightly redesigned version known as the P-63 Airacobra was built explicitly as a 'Lend-Lease fighter', mostly for the Soviet Union. None served with US forces except in secondary roles.

The prototype XP-39 was first flown in April 1939 and contracts were made for the French though in the event these aircraft were taken over by the British as the Airacobra Mk. I with the 37-mm cannon replaced by a 20-mm version. For the USAAF, production P-39Cs entered service in 1941 though the first actual operational version was the P-39D which became available shortly after. Subsequent variants included upgraded engines: the P-39J with a V-1710-59, the P-39K with a -63 and the P-39M with a -83. The last and most produced variants were the P-39N and Q which had V-1710-85 engines and generally improved perfomance. A successor, the P-63 Kingcobra was very similar to the P-39 but designed for export via Lend-Lease. Variants included the main P-63A and P-63C which only differed in terms of powerplant and the RP-63A, B, and G used by the USAAF as manned target aircraft with toughened skin.