In 1922, Curtiss received a contract to develop two examples of a night bomber intended to replace the Martin-designed NBS-1 biplane bomber that was at that time the only twin-engined bomber in Army service. They were assigned the designation XNBS-4.
Curtiss had built 50 Martin-designed NBS-1s for the US Army Air Service in 1921/22, and the XNBS-4 design drew heavily on that experience. Like the NBS-1, the XNBS-4 was a conventional biplane powered by two Liberty 12 engines mounted inside nacelles attached to the lower wing. A crew of four could be carried. However, the XNBS-4 differed from the NBS-1 in having a welded steel-tube fuselage instead of a wooden one. A new and thicker Curtiss C-72 airfoil replaced the RAF 15 airfoil of the NBS-1. The bomb-aimer's position was built into an offset on the port side of the fuselage instead of under the nose gunner's position as was the usual position. Unlike the NBS-1, the XNBS-4 had a boxlike biplane tail, which was somewhat of a retrograde step.
The serial numbers of the two XNBS-4s were 68571 and 68572. The first XNBS-4 was delivered in May of 1924.
One of the XNBS-4s was modified by having each engine nacelle extended aft of the wing trailing edge so that it could accommodate a defensive gunner position in the rear. It was hoped that this arrangement would offer a clearer field of fire for the gunners than the more conventional fuselage-situated positions. This was not an entirely new and unique idea, having been tried out on both British and German bombers during the War.
Like the Elias XNBS-3, the Curtiss XNBS-4 offered no real advantage over the existing Martin-designed NBS-1, and was not ordered into production. However, it did provide Curtiss with valuable design experience which culminated in the successful Condor bomber and transport series of 1927-29.
Specification of the Curtiss XNBS-4:
Two 435 hp Liberty 12A liquid-cooled engines. Maximum speed 100 mph at sea level. Cruising speed 82.9 mph. Landing speed 53 mph. Service ceiling 11,100 feet, absolute ceiling 13,000 feet. Initial climb rate 283 feet per minute. An altitude of 5000 feet could be attained in 11.5 minutes. Range was 500 miles with 2000 pounds of bombs. Empty weight 7864 pounds, gross weight 13,795 pounds. Wingspan 90 feet 2 inches, length 46 feet 5 inches, height 15 feet 9 inches, wing area 1524.4 square feet. Armed with five 0.3-inch Lewis machine guns, two each in positions in the nose and aft cockpits, and a single Lewis firing downward from the bottom of the fuselage. A bombload of 2100 pounds could be carried.