In developing this aircraft, the Bell designers came up against two global problems. The first was that the P-39 was intended for export, and there was no way round this. The impossibility of fitting a turbo compressor to the low-altitude Allison engine (for fully explicable reasons) was one reason why the aircraft was never able to achieve a worthy place in the US air forces. The nature of air combat, both in Europe and the Far East, prevented the P-39 making use of its advantages, while it was not possible to rectify its main drawback — its poor high-speed performance — because of the ban on exporting turbo compressors outside the USA. The second global problem was connected to a technical failure in the production of the 20 mm Hispano-Suiza AN-M1/M2C cannon by the Bendix company. The virtually unfit for purpose 20 mm cannon were replaced by the old, heavy but more reliable Colt-Browning M4
The P39K differed from the earlier models in its engine, the Allison V-1710-63, which was successfully tested in that same D-2. It was fitted with an afterburner system. It developed a nominal power of 1352 h.p. and 1550 h.p. in WEP (War Emergency Power) mode. The K series was the only one fitted with such an engine along with a three-bladed Aeroprop propeller.