<p>In 1930 the Army Air Corps elected to discontinue the two categories which distinguished the light (LB) and heavy (HB) bomber series in favor of a single category (B). Consequently, several of the Huff Daland/Keystone designs, which had been the mainstay of the bomber force, were redesignated in the new series. From the LB-3, the Huff Daland/Keystone range had been twin-engined. Engine changes usually brought about designation changes, and some of the series had single fins in place of the earlier twin fin arrangement.
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<p>A single LB-6 had been completed with two R-1750-1 engines to become the LB-10, and this was ordered into production as the LB-10A. The engines of the production aircraft were Wright R-1690-3s, and the type had a single fin. Sixty-three (30-281/30-343) were bought as B-3As following the category redesignation.
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<p>The designation LB-13 originally applied to seven aircraft, but in the end only five of these were completed with the original engine (R-1860-7) to become Y1B-4s (30-344/30-348). These led to a production order for 25 B-4As (32-117/141). The remaining pair of LB-13s was completed to Y1B-6 standard with Wright R-1820-1 engines, joined by the conversion of three B-3As. The service test aircraft (30-349/353) led to the procurement of 39 B-6As (32-142/32-180).
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<p>The last in the series to be affected by the recategorization was the LB-14, of which three were delivered with Wright R-1750-3 engines as the Y1B-5. Twenty-seven production aircraft followed.
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<p>Keystone bombers initially served only with the 2nd Bomb Group, but the gradual enlargement of the USAAC saw them entering service with 7th and 19th Bomb Groups to form the backbone of the bomber force. They also served overseas in Hawaii, the Philippines and the Panama Canal Zone.
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<hi style=hdr1>Specification</hi>
<hi style=hdr2>Keystone—B-4A</hi>
<list style=aflst>
<item><hi format=bold>Type:</hi> five-seat bomber
<item><hi format=bold>Powerplant:</hi> two Pratt & Whitney R-1860-7 radial piston engines, each rated at 575 hp
<item><hi format=bold>Performance:</hi> maximum speed 121 mph; service ceiling 14,000 ft; range 855 miles
<item><hi format=bold>Weights:</hi> empty 7,951 lb; maximum take-off 13,209 lb
<item><hi format=bold>Dimensions:</hi> wing span 74 ft 8 in; length 48 ft 10 in; height 15 ft 9 in; wing area 1,145.0 sq ft
<item><hi format=bold>Armament:</hi> three 0.3-in Browning machine guns in nose, dorsal and ventral positions; up to 2,500 lb of bombs
