The United States Army Air Corps (USAAC) issued a specification for a specialized bomber trainer, ordering two prototypes from Fairchild Aircraft. The XAT-13 powered by two 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.[2] 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.[3]
The second prototype, designated the XAT-14 was similar in layout but was powered by two 520 hp (388 kW) Ranger V-770 inline inverted 12-cylinder vee-type engines. It first flew in late 1942.[4] 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.[5] 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.[3]
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.[5]
The AT-21 proved unsuitable for use as a trainer due to vibration and oscillation tendencies, as well as an inherent instability caused by the short distance between the rudders and the gull wing. The aerodynamic instability resulted in unacceptable yaw when even slight rudder movements were made.[6]
Deemed unsuitable for its original purpose, the AT-21 was evaluated as an advanced pilot trainer. This also did not work out well, due to poor single-engine performance and multiple landing gear problems. The aircraft was withdrawn from service in 1944 and was replaced in training by examples of the actual aircraft in which the gunners would eventually serve. Many AT-21s were then relegated to target-tow duties.[7]
A small number of AT-21s survived as civilian examples, with one (s/n 42-48053 owned by Craig Cantwell) still in existence in North Texas awaiting restoration.[8]
See main article: Fairchild BQ-3. In October 1942, the U.S. Army Air Forces ordered two XBQ-3 "aerial torpedoes", based on the AT-21 design. Optionally piloted for flight testing and intended to carry of explosive, the XBQ-3 first flew in July 1944, but the project was cancelled later that year.[9]