25 P-39Ds were fitted with the 1100 hp V-1710-59 engine with automatic boost control and were redesignated P-39J.
Serials were 41-7053/7056 and 41-7059/7079.
They differed from the D in having a different engine, the 1100 hp V-1710-59 (E12) with an automatic manifold pressure regulator.
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.