A number of the Keystone aircraft became one off flying testbeds for various newly developed geared or direct drive versions of the Hornet and Cyclone engines. This pro¬gramme gave rise to the new designations LB 8, LB 9, LB 10, LB 11, LB 11A and LB¬-12. Most of these were conversions from existing LB 6 or LB 7 bombers.
The major production model which follow¬ed was the single tailed LB 10A, powered originally by 525 hp R 1690 3 Hornets, of which 63 were built. These were delivered under the new designation B 3A, the USAAC having adopted a single ‘B’ category for all types of bomber in 1930. Later, 27 of them were refitted with R 1750 3 Cyclones in place of the Hornets and were redesignated B 5A.

LB 10A
Engine: 525 hp R 1690 3 Hornet
Span: 22.78 m (74 ft 9 in)
Length: 14.88 m (48 ft 10 in)
Gross weight: 5875 kg (12 952 1 b)
Maximum speed: 183 km/h (114 mph).

The Keystone LB series had been characterized by tapered wings, but the XLB 6 of 1927 (converted from the tenth LB 5) introduced constant chord wings with new powerplant (two 525 hp Wright R 1750 1 Cyclone radials) mounted between the wings, and a lengthened fuselage. Thus modified, this became the standard configuration for the remaining Keystone LB types, beginning with 17 production LB 6s and continuing with 18 LB 7s, which were identical except for their 525 hp Pratt & Whitney R 1690 3 Hornet engines.

Huff Daland’s XB 1 bomber design of 1926 was cancelled and reordered as the Keystone XB 1B, a five seat biplane with two 600 hp Curtiss V 1570 5 Conqueror engines. It never progressed beyond prototype stage.