Huff Daland's single-engined LB-1 Pegasus had proved to be unsuitable in service, so the company revised it as a twin. Using the LB-1 airframe, the XLB-3 (27-333) featured a pair of air-cooled and inverted 420-hp V-1410-1 Liberty engines above the lower wings. This powerplant proved temperamental, so the XLB-3A was re-engined with 410-hp Pratt & Whitney R-1340-1 Wasp engines. The redesign to twin-engined arrangement allowed the provision of nose gunner and bombardier stations, raising the crew complement to five.

Before the XLB-3A had been delivered, Huff Daland produced the XLB-5 (26-208), which was similar apart from being powered by standard Liberty engines, followed by 10 production LB-5s (27-335/27-344). These featured a single large rudder with two smaller rudders to the sides, and were the last in the series delivered as Huff Dalands.

A company reorganization introduced the name Keystone in time for the delivery of 25 LB-5As (28-001/28-025), which differed by having twin fins. These served primarily with the 2nd Bomb Group, and a few reached the 5th Composite Group in Hawaii.