The remaining 25 aircraft on the initial P-39F order were delivered as P-39Js. They differed from the F in having a different engine, the 1100 hp V-1710-59 (E12) with an automatic manifold pressure regulator. Serials were 41-7053/7056 and 42-7059/7079. These
serials were from the original P-39D batch.
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.