Boeing’s B-47 Stratojet was built in larger numbers than any other multi-engine jet bomber and had a revolutionary influence on aircraft design. The Seattle company was the only bomber builder with its own high-speed wind tunnel, giving Boeing an advantage in adopting new aerodynamic technology.
Model 432, submitted in the Air Force bomber competition in December 1944, had a conventional straight wing with jet engines buried within the fuselage, and won a letter contract for design development on February 1, 1945. Captured German research data on the high-speed advantages of swept-back wings was verified and incorporated in the layout of Boeing’s Model 448. That September 1945 proposal had a swept-back wing and six engines, but the enclosed engines were unsatisfactory.
Model 450 replaced this project in October, with four engines paired in pods below and forward of the wing, where interference with airflow over the wing would be minimized. The other two engines were in individual pods near the wing tips. After the mockup was approved in April 1946, construction of two XB-47 prototypes began in June, although contract negotiations were not approved until July 10, 1947.
The XB-47 wing had a Boeing 145 laminar airfoil, a sweepback of 35 degrees, and an aspect ratio of 9.6. Like the XB-48, there were six J35 engines, tandem landing gear with four main wheels, and the pilot and co-pilot sat in tandem under a bubble canopy. A bombardier-navigator in the nose was to get a radar bombing system whose antenna bulged below in a plastic fairing, and the bomb bay could accommodate up to 16 1,000-pound or one 22,000-pound bomb. As was customary, no tactical equipment was provided in the prototypes.
Flight tests on the XB-47 began on December 17, 1947, the 44th anniversary of the Wrights’ first flight. Robert Robbins flew the prototype from Boeing Field to Moses Lake AFB. Tests continued at Muroc proved the Stratojet the fastest bomber in the world, even if the expected 635 mph wasn’t reached. When the second XB-47s was flown July 21, 1948, with J47-GE-3 power plants, top speed did pass the 600-mph level. Eighteen JATO rockets in the rear fuselage accelerated takeoff, and a 32-foot diameter ribbon parachute was used to slow landings.
Cautiously, the USAF put the Stratojet into production at Wichita, Kansas, while reserving its main funds for the B-36. Ten B-47As were ordered October 28, 1948, and the first 88 B-47B aircraft were added to the contract November 14, 1949. First flown June 25, 1950, the day the Korean war began, the B-47A had 5,200-pound-thrust J47-GE-11s, increased takeoff weight, and was used for service tests and training. Only four had K-2 bombing systems, and two had a pair of .50-caliber guns mounted in the tail cone; one testing an Emerson A-2 radar aiming system, and the other the General Electric A-5 system.
The Korean War opened the national purse and the North Atlantic Treaty provided bases abroad to encircle the potential enemy. To carry out the largest bomber-production program since World War II, Stratojet production by Boeing-Wichita was joined in December 1952 by B-47s from the factories in Tulsa, Oklahoma, and Marietta, Georgia, reopened under the management of Douglas and Lockheed. Far more complex than the B-29, the B-47 required major innovations in metal work and wiring.
Extending the B-47B’s range was the first task, for the Stratojet needed more than twice the fuel to cover the same distance as a B-29. A pair of 1,780-gallon drop tanks under the wings increased fuel capacity to 15,213 gallons, and a receptacle for in-flight refueling from a flying boom tanker was provided in the nose. A periscope sight for a K-4A bombing system replaced the B-47A’s Plexiglas nose. A shorter bomb bay allowed more fuel space, but still accommodated 18,000 pounds of Mk 6 nuclear, or conventional bombs, although tactical plans emphasized the former.
The B-47B first flown April 26, 1951, had the same J47-GE-11s, but J47-GE-23s were used after the first 88 aircraft. Early B models were mostly used for training, and lacked the B-4 radar system for the two .50-caliber M-3 guns with 1,200 rounds provided for rear defense. Since these jet bombers had much longer takeoff and landing runs than the B-29s they replaced, they needed 18 solid-fuel JATO rockets to reduce the takeoff ground run from 9,100 to 7,200 feet.
Strategic Air Command’s 306th BW received its first B-47B on October 23, 1951, but that Wing would not become operational for another year. B-47s were to be deployed in 45-plane medium-bomber Wings, each accompanied by an air-refueling squadron with 20 KC-97 tankers. Air refueling transformed the B-47 into an intercontinental bomber.
The three-man crews had to learn entirely new and more complicated skills than those required for the B-29. Until 1958, Air Training Command had the task of preparing those men for their new roles in unarmed B-47Bs.
Of 397 B-47Bs delivered by June 1953, Lockheed assembled eight, and Douglas ten. Many B-47Bs were modified to B-47E standards (as the B-47B-II or III) in 1953/54, 66 became TB-47B trainers, and 24 became RB-47Bs when camera pods were installed in bomb bays. The first YRB-47B joined the 91st Strategic Reconnaissance Wing in April 1953.
The 88th B-47B was first redesignated XB-56, and then YB-47C, based on a January 1950 design to increase range by replacing the six jet engines with four Allison T71-A-5 engines. That aircraft was canceled in December 1952 before its completion because the power plants were unready.
Two XB-47Ds were actually B-47Bs completed as unarmed test-beds for a pair of 9,710-hp Curtiss-Wright YT49-W-1 turboprops in the inboard engine pods, while the outboard pods kept their J47s. First flown on August 26, 1955, and reported to have a 597-mph top speed at 13,500 feet, it may have been the fastest propeller-driven USAF aircraft.
The principal Stratojet model was the B-47E, which had engine water injection to increase their J47-GE-25A’s thrust and 33 jettisonable JATO units for takeoff. Ejection seats, K-4A bombing system and a stronger landing gear were provided. First flown January 30, 1953, the E added 845 pounds of chaff, to confuse hostile radar, to the 18,000-pound bomb load.