In 1936, some rich residents of Long Island, such as E. Roland Harriman, asked Grumman to build them an aircraft. They would use this aircraft to fly to New York City. The G-21 was designed after this. It was an all-metal, high-winged monoplane powered by two Pratt & Whitney R-985 Wasp Jr. nine-cylinder, air-cooled radial engines mounted on the leading edge of high-set wings. The deep fuselage served also as a hull and had hand-cranked retractable landing gear. First flight of the prototype took place on May 29, 1937.