The Pratt & Whitney R-1535 Twin Wasp Junior was an engine used in American aircraft in the 1930s. The engine was first introduced in 1932 as a 14-cylinder version of the 9-cylinder R-985.[1] It was a two-row, air-cooled radial design. Displacement was 1,535 in³ (25.2 L); bore and stroke were both 5-3/16 in (131.8 mm).