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. This was the first problem the Bell specialists tried to tackle. The Bell specialists initially tried to solve this problem themselves. The virtually unfit for purpose 20 mm cannon were replaced by the old, heavy but more reliable Colt-Browning M4. The cannon had a lot of faults, mainly its low rate of fire and extremely inadequate stock of ammunition - only 30 shells. On the other hand, in spite of its weight, it could easily be fitted in the front part of the Cobra’s fuselage; furthermore, it has one incomparable advantage over the Bendix product - it did actually fire.
It must be noted that the M1 cannon were not always fitted to the first series of the P-39, you could find the M4 here and there. But only the Model 26 (P-39G) carried the M4 as standard equipment. However, it did not go into production, but became the founder of a whole number of virtually identical series from P-39K to P-39Q. The main units were borrowed from the quite promising P-39D-2 series.
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. It was a full 360 kg heavier than the D-2, although externally, apart from the propeller, there was no visible difference. In fact, the extra weight was due to additional armor plating, which was used beginning with the K and on all subsequent series.
Altogether, by August 1941, 210 examples of the series K-1-BE, K-2-BE and K-5-BE, which were virtually identical, were built Forty of them were sent to the USSR.