<p>Heavy firepower had been a distinguishing feature of the fighter aircraft which faced each other in Europe when World War II began, and progressive development aimed to increase this as much as possible. To neutralize the advantage held by attacking fighters, power-operated multi-gun turrets were evolved, to provide fast aiming and ranging of their concentrated firepower. One operational British fighter, the Boulton Paul Defiant, was even provided with a gun turret in this category for attack, rather than defense.
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<p>The importance of these requirements were not at first appreciated by the US Army: the B-17B Fortress, for example, was protected by only five machine guns in five separate mountings. When the significance was understood, as combat experience in Europe became available to US Army Air Corps planners, steps were taken immediately to ensure that adequate offensive and defensive firepower, including the introduction of power-operated turrets, was written into procurement specifications. These steps did not, however, cover deficiencies in the training schedule: not only were there no air gunners with the experience to use a gun turret, if and when provided, but there were also no specialized air gunnery training schools. Neither were there any suitable aircraft in which pupils could gain the essential air-to-air firing practice.
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<p>To resolve this latter shortcoming, the USAAC ordered two specialized gunnery training prototypes from the Fairchild Engine & Airplane Corporation. The first XAT-13 was intended to serve for the training of all members of a bomber's crew working as a team, and the single prototype (41-19500) was powered by two 600-hp (447-kW) Pratt & Whitney R-1340-AN-1 radial engines. The second XAT-14 prototype (41-19503) was powered by two 520-hp (388 kW) Ranger V-770-6 inline engines and was generally similar in layout, but was adapted subsequently as a more specialized trainer for bomb aimers under the designation XAT-14A, with its defensive guns removed. Testing of these aircraft served to crystallise ideas, resulting in the procurement of a special gunnery trainer under the designation AT-21 and given the name Gunner.
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<p>Of the 175 AT-21s constructed, 106 were built by Fairchild and, to speed deliveries to the USAAF, 39 were built by Bellanca Aircraft Corporation and 30 by McDonnell at St Louis. Entering service with newly established air gunnery schools, they remained in service until 1944, displaced eventually by the production of training examples of the operational type in which the air gunners would eventually serve. One was tested as a remotely-controlled aerial bomb under the designation XBQ-3.
<item><hi format=bold>Powerplant:</hi> two 520-hp (388-kW) Ranger V-770-15 inline piston engines
<item><hi format=bold>Performance:</hi> maximum speed 225 mph (362 km/h) at 12,000 ft (3,660 m); cruising speed 196 mph (315 km/h) at 12,000 ft (3,660 m); service ceiling 22,150 ft (6,750 m); range 910 miles (1,464 km)
<item><hi format=bold>Weights:</hi> empty 8,654 lb (3,925 kg); maximum take-off 11,288 lb (5,120 kg)
<item><hi format=bold>Dimensions:</hi> span 52 ft 8 in (16.05 m); length 38 ft 0 in (11.58 m); height 13 ft 1 in (4.00 m); wing area 378 sq ft (35.12 m<su>2</su>)
<item><hi format=bold>Armament:</hi> one 0.30-in (7.62-mm) machine gun in fuselage nose and two 0.30-in (7.62-mm) guns in power-operated dorsal turret
