The Keystone XLB-6 was obtained by fitting the 10th LB-5 (serial number 27-344) with new straight-chord wings of 75-foot span. These wings were untapered and were slightly swept back. The twin Liberty engines of the LB-5 were replaced by a pair of 525 hp Wright Cyclone radials. The engines were suspended between the wings rather than resting on top of the lower wing.
The XLB-6 aircraft was in competition with the Curtiss XB-2 for production in early 1928. Although the Curtiss design was clearly superior, the USAAC leadership was quite conservative and ordered the LB-6 into production, ordering 18 aicraft with one to be converted to XLB-9 configuration before delivery. Later in the year, the Army reconsidered the merits of the Curtiss XB-2 and decided to order 12 production aircraft to be built as B-2s
Seventeen production LB-6 aircraft were delivered by Keystone to the USAAC between August and September of 1929. Serials were 29-011/027. All LB-6s were identical to the XLB-6 except for minor refinements and a revised angular shape for the twin rudders. Additionally, one LB-7 (29-010) was converted to an LB-6 at Wright Field after being tested as the XLB-9. Three of the LB-6s (29-013, 29-014, and 29-016) were converted to LB-7s at France Field in May 1930.
The LB-6 served with the 2nd Bomb Group based in the US as well as with the 5th Composite Bomb Group based in Hawaii. The LB-6 had a better performance than the Liberty-powered LB-5. It was ten miles per hour faster and had twice as fast a climb rate. The LB-6 was known as Panther by the Keystone company, although this was not an official USAAC name. The LB-6s rapidly became obsolete, and all surviving examples were withdrawn from service and scrapped in 1933-35.
Specification of the Keystone LB-6:
Two 525 hp Wright R-1750-1 Cyclone alr-cooled radial engines. Maximum speed 114 mph at sea level, 106 mph at 5000 feet. Cruising speed 95 mph. Landing speed 58 mph. Service ceiling 11,650 feet, Absolute ceiling 14,000 feet. Initial climb rate 600 feet per minute An altitude of 5000 feet could be attained in 10.3 minutes. Range 632 miles with 2003 pounds of bombs. Weights: 7024 pounds empty, 12,155 pounds gross. Wingspan 75 feet, length 43 feet 5 inches, height 18 feet 1 inches, wing area 1148 square feet. Two Lewis machine guns in an open gunner's position in the nose, two Lewis machine guns in an open dorsal gunner's position, one Lewis gun firing downward through an opening in the lower fuselage.
29-011 DELIVERED 5/1/29, REDESIG. ZLB-6 WITH 40TH SCHOOL SQ. ON 3/7/31, SURVEYED 2/6/35. 29-012 DELIVERED 7/16/29, REDESIG. ZLB-6 WITH 40TH SCHOOL SQ. ON 4/15/31, WRECKED 8/19/31. 29-013 DELIVERED 7/21/29, CONVERTED TO LB-7 5/14/30, WRECKED 6/19/31. 29-014 DELIVERED 7/26/29, CONVERTED TO LB-7 5/14/30, SURVEYED 11/2/33. 29-015 DELIVERED 7/26/29, WRECKED AND SURVEYED ON 10/12/29. 29-016 DELIVERED 7/21/29, CONVERTED TO LB-7 5/14/30, WRECKED 6/19/31. 29-017 DELIVERED TO HAWAII 7/11/29, SURVEYED IN HAWAII 2/25/35. 29-018 DELIVERED TO HAWAII 7/22/29, SURVEYED IN HAWAII 2/25/35. 29-019 DELIVERED TO HAWAII 7/26/29, SURVEYED IN HAWAII 4/8/35. 29-020 DELIVERED TO HAWAII 8/1/29, SURVEYED IN HAWAII 2/25/35. 29-021 DELIVERED TO HAWAII 8/7/29, SURVEYED IN HAWAII 5/16/35. 29-022 DELIVERED TO HAWAII 8/15/29, SURVEYED IN HAWAII 5/18/33. 29-023 DELIVERED TO HAWAII 8/20/29, SURVEYED IN HAWAII 2/25/35. 29-024 DELIVERED TO HAWAII 8/26/29, SURVEYED IN HAWAII 5/10/34. 29-025 DELIVERED TO HAWAII 9/4/29, SURVEYED IN HAWAII 9/26/33. 29-026 DELIVERED 10/29/29, CONVERTED TO LB-11 3/3/31, CONVERTED TO LB-11A 4/1/31, CONVERTED TO LB-6 5/31/32, REDESIG. ZLB-6 WITH 40TH SCHOOL SQ. ON 6/2/33, SURVEYED 9/29/34. LB-7 29-010 Delivered 12/27/29, CONVERTED TO XLB-9 3/2/31, CONVERTED TO LB-6 12/4/31, EDESIG. ZLB-6 WITH 40TH SCHOOL SQ. ON 3/16/32, SURVEYED 9/26/33.