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In 1931, the V-770 design was put to paper, based on the Ranger 6-440 series of inline air-cooled engines, and test flown in the Vought XSO2U-1 Scout. In 1938 it was tested in the Curtiss SO3C Seamew and found to be unreliable with a tendency to overheat in low speed flight. By 1941 a more developed V-770 was installed in the Fairchild XAT-14 Gunner prototype and found satisfactory for the production Fairchild AT-21 Gunner gunnery school aircraft.

Produced from 1941-1945, the V-770 featured a two-piece aluminum alloy crankcase, steel barreled cylinders with integral aluminum alloy fins and aluminum alloy heads. The V-770 was one of very few V-type, in-line, air-cooled engines to reach production. The engine was used in a relatively small number of Army Air Forces aircraft, among them the Fairchild AT-21 twin-engine trainer of which approximately 175 were built, and in the two Bell XP-77s.

The United States Army Air Forces (USAAF) laid out a specification for a specialized bomber trainer, ordering two prototypes from Fairchild Aircraft. The XAT-13 powered by two 450 hp (336 kW) Pratt & Whitney R-1340-AN-1 radial engines, emerged as a "scaled down" bomber with a single machine gun in the glazed nose and a top turret with twin machine guns and fitted with tricycle landing gear. The concept was to have a single type that was able to duplicate the bomber crew positions from piloting, navigation, bomb aiming/dropping to aerial gunnery.

The second prototype, designated the XAT-14 was similar in layout but was powered by two 520 hp (388 kW) Ranger V-770 inline engines. The Ranger V-770 is an inverted 12 cylinder vee type aero engine. A further refinement to suit the aircraft for bombardier training with the nose gun and turret removed, led to a new designation, the XAT-14A. Both prototypes featured an unusual construction, being built from "Duramold" plastic-bonded plywood. At the end of the testing period, the USAAC ordered the inline version as the AT-21 Gunner, specialized for gunnery training. The AT-21 had a crew of five, pilot, co-pilot/gunnery instructor and three pupils.

Fairchild Aircraft Company built one aircraft in Hagerstown and 106 aircraft at their Burlington, North Carolina plant in 1943 and 1944, while Bellanca Aircraft built 39 at New Castle, Delaware, and McDonnell built 30 aircraft at their St. Louis plant, both companies were enlisted to speed production and delivery to training units.