The Keystone B-4A Panther was ordered alongside the B-6A, and together they were the last biplane bombers to enter American service. Later in the 1930s the entire Panther range would have been given a single B designation with the changes of engine indicated by model letters, but in the early 30s minor changes were often given new designations. The B-3 and B-4 were both powered by Pratt & Whitney engines, the B-5 and B-6 by Wright engines. The B-3 and B-5 of 1930 both had 525hp engines, the B-4 and B-6 of 1932 both had 575hp engines.
Like all of the Panthers the B-4A was armed with three 0.30in guns, one in the nose, one in the rear cockpit and one in a fuselage tunnel. The B-4A carried the same bomb load as the earlier B-3A, and had the same operational range. The more powerful engines did give it a slightly increased top speed, but by the time the B-4A entered service in 1932 the Martin B-10 was already under development, and this modern all-metal monoplane would make the Panther obsolete - it had a maximum range twice that of the B-4A and a top speed of 215mph.
Five Y1B-4 service test aircraft were produced by modifying aircraft first ordered as the LB-13. They were then followed by twenty-five production aircraft, which entered service in 1932. A number of the B-4As were used during the Air Corps’ brief airmail experiments during the first half of 1934, but the bombers were not overly successful in that role – the presence of the heavy cargo loads in unexpected placed affected the balance of the aircraft. By the mid 1930s the B-4A was obsolete as a bomber, but some remained in use as observation aircraft into the 1940s.
Engine: Pratt & Whitney R-1860-7 Hornet B
Span: 74ft 9in
Length: 48ft 10in
Height: 17ft 2in
Empty Weight: 7,951ft
Gross Weight: 13,209ft
Maximum Speed: 121mph
Range: 855 miles
Guns: Three 0.30in Browning machine guns
Bomb load: 2,496lb max, 1,995lb standard