In 1937, at the behest of the United States Army Air Corps, the Allison Engine Company agreed to design and build a large-displacement high-power aircraft engine.

The resulting V-3420 was essentially a pair of 12-cylinder Allison V-1710 engines mated to a common crankcase with a 30° angle between the inner cylinder banks.

The crankshafts of the two V-1710 engines were geared together to drive a common propeller shaft.

It was envisioned as a powerful yet compact engine for several advanced USAAF projects of the day, including the Douglas XB-19, the Boeing XB-39 Superfortress, the Lockheed XP-58 Chain Lightning, and the Fisher P-75 Eagle.

[citation needed] Data from Aircraft Engines of the World 1946 Related development