The Lycoming XR-7755 was the largest piston aircraft engine ever built in the United States,[Note 1] with 36 cylinders totaling about 7,750 in3 (127 L) of displacement and a power output of 5,000 horsepower (3,700 kilowatts).

In one final attempt, Lycoming decided to go all out and build what would turn out to be the largest displacement aircraft piston engine in the world.

They put together a team under the direction of VP of Engineering Clarence Wiegman at their main Williamsport factory in the summer of 1943 and started work.

The propeller reduction gearing also had two speed settings to allow for a greater range of operating power than adjustable props alone could deliver.

However, by this time the Air Force had lost interest in new piston designs due to the introduction of jet engines.