The next production version of the Airacobra was the P-39F (Bell Model 15B). It was almost identical to the P-39D, but had an Aeroproducts constant-speed propeller in place of the original Curtiss-Electric, because the Curtiss-Electric propellers could not be delivered in sufficient quantities. The F also differed externally from the D in having twelve exhaust stacks on each side of the fuselage rather than six.
Initial orders, placed on September 13, 1940 under contract AC 15675, called for 229 aircraft. Serials were 41-7116/7344. 27 Fs were modified in the field to become P-39F-2s with the addition of belly armor and the fitting of cameras to the rear fuselage to suit the aircraft for ground support duties.
There are reports that a single P-39F was experimentally modified as an advanced trainer with a second cockpit ahead of the original one. All armament was removed, and dual controls were fitted. The designation given to this odd-looking aircraft was TP-39F.
The last 25 P-39Fs were fitted with the 1100 hp V-1710-59 engine with automatic boost control and were redesignated P-39J. Serials were 41-7043/7056 and 41-7059/7079.
At this stage of the war, the production block system of subdividing model changes was not yet in effect, and relatively minor changes brought about letter (series) changes in the model. For example, in the Model P-39, minor changes in the D series resulted in the F, G, K, L, M, and N series rather than new blocks of the D
Serials of P-39F-1 Airacobra
41-7116/7344 Bell P-39F-1 Airacobra 7123,7177,7182,7183,7226,7230,7248,7266,7270/7272,7278, 7294,7295,7299,7302,7303,7305,7309,7310,7312,7318,7325, 7326,7332,7334,7339 modified as P-39F-2.