Keystone XLB-3

The Keystone XLB-3 (originally built under the Huff-Daland name) was a prototype bomber biplane developed in the United States in the late 1920s. It was a twin-engine development of the single-engine LB-1, brought about by a change in policy by the USAAC.

Design and development
The shift from a nose-mounted engine to engines mounted in nacelles on the lower wing created an opportunity to provide stations for two extra crewmembers: a bombardier and a nose-gunner, bringing the total to five. The LB-1's single tailfin and rudder was augmented by an extra rudder either side of it.

Operational history
A single prototype was constructed, and delivered to the USAAC for evaluation at the end of 1927. Evaluation, however, showed that performance was actually inferior to that of the single-engine LB-1. The decision was taken to change the XLB-3's water-cooled Liberty engines for air-cooled radials, at which point it was redesignated XLB-3A. With performance still unsatisfactory, development was abandoned in favor of a parallel design, the LB-5.

Variants

 * XLB-3 - original version with Allison VG-1410 air-cooled inverted V-12 engines (1 built)
 * XLB-3A - version with Pratt & Whitney R-1340 radial engines (1 converted from XLB-3)