The XB-47 was the third of four design proposals approved by the Army Air Force in late 1944. The other designs resulted in the North American XB-45, Convair XB-46 and Martin XB-48. The XB-47 was designed to use the newly developed General Electric J35 turbojet. The Army specification required the proposed aircraft to be capable of 550 mph maximum speed, a service ceiling above 40,000 feet and a combat radius in excess of 1,000 miles.
The initial Boeing designs were based on the basic B-29 airframe, but post-war research investigating swept wing designs (and captured German data) caused a radical change in the project. The initial designs were dropped and a new design featured a 35-degree swept wing and six jet engines in four nacelles. The two inboard engine nacelles each housed a pair of engines while the outboard nacelles had only one. Boeing test data showed that forward-protruding nacelles allowed the best airflow to the engines while minimizing turbulent airflow over the wings (decreasing lift). The basic nacelle design became a standard characteristic of Boeing military and commercial jet designs which continues to the present day.
The XB-47 had an unusual tandem main landing gear arrangement. The forward gear was significantly higher than the aft gear to increase the angle of attack and improve take off performance. Auxiliary outrigger landing gear was installed in each inboard engine nacelle to steady the aircraft during taxiing and while parked.
Two XB-47s were ordered in the spring of 1946 and the initial prototype's first flight was Dec. 17, 1947. The second XB-47 was completed with more powerful General Electric J47 jet engines and first flew on July 21, 1948. The J47 became the standard for all production versions of the Stratojet. The first XB-47 was refitted with J47s after its initial series of flight tests.
Six-engine swept wing jet bomber
TECHNICAL NOTES: Armament: Two radar directed .50-cal. machine guns in a tail turret and up to 22,000 lbs. of bombs (one 22,000-lb. "Grand Slam" or 16 1,000-lb. GP bombs maximum) Engines: Six General Electric J35-GE-7 axial flow turbojets of 4,000 lbs. thrust each; provisions for 18 1,000-lb. thrust rockets for RATO takeoffs; second XB-47 had six General Electric J47-GE-3 axial flow turbojet engines of 5,200 lbs. thrust each (the first aircraft was refitted with J47s) Maximum speed: 578 mph at 15,000 ft. Cruising speed: 466 mph Range: 2,650 miles with 10,000 lbs. of bombs (estimated) Service ceiling: 37,500 ft. Span: 116 ft. 0 in. Length: 107 ft. 6 in. Height: 27 ft. 8 in. Weight: 162,500 lbs. (maximum takeoff weight) Crew: Three (pilot, copilot-radio operator-gunner, bombardier-navigator) Serial numbers: 46-065 and 46-066