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.
Sources:
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War Planes of the Second World War, Fighters, Volume Four, William Green,
Doubleday, 1964.
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The American Fighter, Enzo Anguluci and Peter Bowers, Orion Books, 1987.
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United States Military Aircraft since 1909, Gordon Swanborough and Peter
M. Bowers, Smithsonian Institution Press, 1989.
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P-39 Airacobra in Action, Ernie MCDowell, Squadron/Signal Publications,
1980
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The Calamitous 'Cobra, Air Enthusiast, August 1971.
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Airacobra Advantage: The Flying Cannon, Rick Mitchell, Pictorial Histories Publishing Company, Missoula, Montana
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Bell Cobra Variants, Robert F. Dorr, Wings of Fame, Vol 10, AirTime Publishing , Inc., 1998.
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E-mail from Phil Marchese on production block system.
