The LB-6 was in competition with the Curtiss XB-2 for production in early 1928 and although the Curtiss aircraft was clearly the better of the two, the conservative Army Air Corps leadership chose the Wright-powered LB-6 and the Pratt & Whitney-powered LB-7, ordering 35 aircraft. The LB-6/LB-7 was the first operational service model of a 13,000 lb twin-tail biplane bomber of a series produced by Keystone. 35 served operationally between 1929 and 1934. A number of variants were built for test and evaluation purposes but never placed into production or service. The LB-10 variant became the basis for the B-3 to B-6 series of similar bombers that were the last biplane bombers ordered by the Air Corps. The Keystone XLB-6 prototype was created by refitting the final triple-tailed Keystone LB-5 with an LB-5A tail unit; longer straight-chord, untapered and noticeably swept-back wings; and 525 hp 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-" 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 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.
Operational history
According to an article in the Baltimore Sun the XLB-12 Prototype was used on a refueling and bombing trial in April 1929. Big Bomber to be Refueled during Flight "The huge twin-motored army bomber which will take off from Dayton, O., fly to New York City and be refueled while in midair, after which she will drop bomb flares on the city and make a return flight to the Dayton army field. The flight is to be a practicable demonstration by the Army Air Corp of the value of refueling airplanes in the air when engaged in bombing expeditions over a long distance". The aircraft was wrecked in August the same year. Eight LB-6s and all operational LB-7s were delivered to the 20th and 96th Bomb Squadrons of the 2nd Bomb Group at Langley Field, Virginia, and the 11th Bomb Squadron of the 7th Bomb Group at Rockwell Field, California, between January and September 1929. The nine remaining LB-6s were sent to the 23rd and 72nd Bomb Squadrons, attached to 5th Composite Group at Luke Field, Hawaii, between July and September 1929. In May 1930 three LB-6s were re-engined to become LB-7s and transferred to the 25th Bomb Squadron of the 6th Composite Group based at France Field in the Panama Canal Zone, where they were all written off in 1931. Beginning in March 1931 the surviving bombers in the Continental United States were taken out of front-line service, re-designated the ZLB-6 and ZLB-7, and sent to the 40th School Squadron at Kelly Field, Texas, where all were scrapped or surveyed by April 1935. The Hawaiian LB-6s served until 1934, when they too were surveyed in-situ.
