The Pratt & Whitney R-4360 Wasp Major is an American 28-cylinder four-row radial piston aircraft engine designed and built during World War II, and the largest-displacement aviation piston engine to be mass-produced in the United States. It was the last of the Pratt & Whitney Wasp family, and the culmination of its maker's piston engine technology. The war was over before it could power airplanes into combat. It powered many of the last generation of large piston-engined aircraft before turbojets, and equivalent (and superior) horsepower turboprops (such as the Allison T56), supplanted it. Its main rival was the twin-row, 18-cylinder, nearly 3,350 cu in (54.9 l) displacement Wright R-3350 Duplex-Cyclone, first run some seven years earlier (May 1937).