The fourteen-cylinder Pratt & Whitney R-1535 Twin Wasp Junior was introduced in 1932 as an alternative to nine-cylinder radials of the same horsepower. It was thought that a fourteen-cylinder radial with a smaller diameter would provide greater performance due to lower profile drag. The diameter of the Twin Wasp Junior was 43 7/8 inches (1114 mm), whereas the Pratt & Whitney Hornet diameter was 55 7/16 inches (1408 mm). However, the increased weight of the twin row, offset any gain from a lower profile. The weight was 1162 lbs. (528 kg) while the Hornet had a weight of 880 lb (400 kg). Although by 1934, the R-1535 produced almost the same power as the single-row R-1690 Hornet,¹ the Grumman F3F would have a faster takeoff and faster climb from a carrier powered with a nine-cylinder Wright R-1820 Cyclone radial than those powered with the R-1535. At altitude and higher speeds, the increase in performance was found to be negligible.

    The only aircraft to take advantage of the small frontal area was not the military but racing aircraft. The most famous example was the Hughes H-1 Racer. Hughes spared no expense for his record making airplane which recorded a top speed of 352 mph (567 km/hr.). The Hughes H-1 racer now resides in the National Air & Space Museum.

    Twin Wasp Junior engines used the same cylinders common to the R-985 Wasp Junior with an original rating of 625 hp at 2400 rpm. The bore was 5.2 in (132 mm) and stroke was 5.2 in (132 mm) with a compression ratio of 6:1. By the following year, power increased to 725 hp. Most of the engines were rated at 700 to 800 hp at 2650 rpm with a maximum of 950 hp for the -64. Unlike the single-row R-985 Wasp Junior which had a two-piece crankshaft and one-piece master rod, the R-1535 had a two-throw crankshaft that was one-piece. This required a master rod that was capped to allow removal. The crankshaft was supported on roller bearings mounted in the front and rear of the crankcase and a ball bearing supporting the center of the crankshaft. Initially the center bearing was giving problems, but this was later corrected.

    A relatively small number of 2,880 Twin Wasp Junior engines were produced from 1931 until 1941.² This was mostly due to Pratt & Whitney's declining interest in the engine. At the time, Pratt & Whitney was also producing the Wasp Junior, the Hornet F and G models, and the Twin Wasp R-1830. They were also doing preliminary work on other promising projects, so something had to give and that was the R-1535.