Development
The Keystone XLB-6 prototype was obtained by refitting the final triple-tailed Keystone LB-5 (serial number 27-344) with an LB-5A tail unit; longer (75 foot) straight-chord, untapered and noticeably swept-back wings; and 525 hp (391 kW) Wright Cyclone R-1750-1 radial engines suspended on struts between the wings rather than mounted on the lower wing. This produced a bomber with twice the rate of climb as the LB-5 and slightly faster cruise speed. Production LB-6s featured a five-foot longer fuselage.
The LB-7 used the same extended-span LB-6 airframe powered by Pratt & Whitney Hornet engines.
The remaining variants were ordered before the 1930 change of designators by the United States Army Air Corps from "LB-" (Light bomber) to "B-", but were delivered after the change. Although delivered as bombers, they saw service as cargo and observation aircraft. The LB-10 variant became the basis of the B-3 through B-6 bombers, all of which were used the original LB-6 design, and which served as the primary bombardment force of the Army Air Corps prior to the advent of the monoplane bomber.
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