The Keystone light bomber had their origins in the Huff-Daland XLB-1 prototype that was bought by the US. Army in 1923 and evaluated with an 800-hp Packard 1A-2540 engine. The followed 10 LB-1 production bomber with the improved 2A-2540 engine. Next came the prototype with two 240-hp liberty V-1410 inlines on the lower wings to allow the relocation of the bombardier to the optimum nose position, this machine was reneged with 410-hp Pratt & Whitney R-1340-1 radials as the XLB-3A. However the XLB-5 prototype reverted to Liberty engines, and the 10 LB-5 bomber had the same power plant. Jest before this Huff-Galand had become Keystone, and the 25 L-5A bombers with a revised tail unit were Keystone rather than Huff-Daland aircraft. The XLB-6 prototype of 1927 introduced new wings and two 525-hp Wright R-1750-1 radials, and was followed by 17 LB-6 bombers. The 18 LB-7s were identical apart from their different engines, and two of these were evaluated as an LB-8 and LB-9 with 550-hp Pratt & Whitney R-1860-3 and 575-hp Wright GR-1750 radials respectively.