Several engineers of the failed Franklin Automobile Company of Syracuse, New York, a manufacturer of air-cooled automobiles, formed Air Cooled Motors in 1935. By 1938 a range of flat-4 and 6 aircraft engines was being produced, retaining the Franklin name.
Installed vertically, with a fan providing additional engine cooling, the O-335-5D powered the Bell TH-13M Sioux helicopter and its civil counterpart, the Model 47.
In 1975, the company disbanded and sold all rights to the Polish government, becoming part of PZL, an association of Polish aero and engine manufacturers. Following the 1989 fall of Communism, the firm became WSK PZL – Rzeszów.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.