History
Background
Development of the B-47 began in 1943 when the US Army Air Forces requested a jet-powered reconnaissance bomber. On November 17, 1944, the Army issued military characteristics for the new bomber, requiring a minimum top speed of 500 mph, an operating altitude of 35,000 to 40,000 feet, a 2,500 to 3,5000 miles range, a takeoff without assistance devices over a 50-foot obstacle on a 5,000 to 5,000-foot runway, and a capability of bombing land and naval targets. Boeing, North American Aviation, Convair, and Martin were among several companies that responded to the request. The USAAF awarded contracts to all four companies requiring North American and Convair development on four-engined designs of B-45 and XB-46. Boeing and Martin were to build six-engined aircraft, the B-47 and XB-48. Their powerplant was to be General Electric’s new TG-180 turbo engines.
Boeing’s initial design was Model 424, a scaled-down version of the piston-engined B-29 Superfortress with four engines. It was later revised to Model 432, with the four engines buried in the forward fuselage. In May 1945, Boeing’s Model 432 was revised with 35-degree swept wings and tail after discovering the secret German aeronautics documents and studies regarding the swept-wing configuration theory and wind-tunnel data. The revised Model 432 was later changed to Model 448, presented to the USAAF in September 1945, adding two more TG-180s in the rear fuselage. The engines were moved to streamlined pylon-mounted pods under the wings, with two TG-180s in a twin-pod mounted on a pylon on each wing, plus another engine at each wingtip. This change was incorporated in an iterated design, Model 450. The Army Air Force expressed approval of this new configuration, and in April 1946, the USAAF ordered two prototypes designated XB-47. Assembly began in June 1947, and the first XB-47 was rolled out on September 12, 1947, a few days before the USAAF became a separate and independent service, the US Air Force, on September 18, 1947.
The XB-47 prototype flew for the first time on December 17, 1947, and the second XB-47 prototype later flew on July 21, 1948. Flight testing of the prototypes was thoroughly methodical, taking notes of any defects and instability issues in the flight testing. By the middle of 1948, the USAF’s bomber competition was completed, and the USAF authorized a formal contract for 10 B-47As on September 3, 1948, the first A flying on June 25, 1950. The non-armed B-47As were restricted as non-combat-ready test aircraft and used for airframe development and crew trainers. The first true production model was the B-47B, the first B flying on April 26, 1951. Modifications were included on the B-47B aircraft, such as air refueling equipment and 0.50-cal. machine guns in the tail turret. A total of 399 B-47Bs were built. Operational service of early B-47 Stratojets first began on October 23, 1951, with the USAF Strategic Air Command. From this time, the modifications and rebuilds continued on the B-47 model.
B-47E Stratojet - Development and Design
The designations B-47C and B-47D applied to variants that were never materialized and produced; thus, the definitive and subsequent variant of the Stratojet began with the B-47E. It differed from its predecessors by having upgraded J47-GE-25 engines with higher thrust and strengthened landing gear to permit higher gross weights. The radar-controlled 20-mm twin cannons were incorporated into the rear tail turret of the B-47E. The B-47E featured a new Clamshell canopy instead of an early sliding canopy.
The first B-47E (51-2357) flew on January 30, 1953, and most B-47Bs were eventually retrofitted to the E standards. Early B-47Es kept the nose windows and internal ATO of the B-47Bs. The later B-47Es had a revised nose section with ejection seats, and the internal Assisted-Takeoff system was replaced with an external jettisonable system. The later B-47Es had upgraded J47-GE-25A engines, capable of 7,200 pounds of wet thrust with the aid of water/alcohol injection.
Major modifications resulted in Roman numeral suffixes being applied to denote aircraft configuration changes. These configurations were as follows:
B-47E-I Modification:
- Operable water injection
- Phase II ECM
- 33 RATO rack
- 200,000-pound gross weight capability
- Aft auxiliary fuel tank (large ATO tank)
- Cycle I bombing/navigational system
B-47E-II Modification:
- Phase XI modifications
- Phase III ECM
B-47E-III Modification:
- 3-Phase AC electric power (constant speed alternators)
B-47E-IV Modification:
- 230,000-pound gross weight capability
- Cycle I CP bombing/navigational system with MA-7 bomb/navigational radar and AN/APS-54 bombing radar
B-47E Stratojet - USAF Service
Along with early B-47 models, the B-47Es were rushed into production and service to meet the USAF requirement during the high-tense stages of the Cold War. The Stratojets served as a backbone of the US Air Force Strategic Air Command through the 1950s and 1960s. In February 1955, the USAF SAC received its first B-47E-IV, and in March 1955, it was decided that all active B-47s would be brought up to the IV standard.
By 1956, the USAF had 28 wings of B-47 bombers and five wings of RB-47 reconnaissance aircraft. The B-47 was the first line of the US’s strategic nuclear deterrent, often operating from forward bases in the United Kingdom, Morocco, Spain, Alaska, Greenland, and Guam, ready to attack the Soviet Union at short notice. By 1957, the B-47 production ceased, and the B-47 fleet continued to serve and retrieve modifications and rebuilds.
While serving in quiet service, the B-47 Stratojet was used as a learning tool for jet bomber operations and as an interim bomber until the B-52 Stratofortress could be placed into service. The B-47 bombers were involved in operational practices for high-altitude bombing to low-altitude strikes to prepare for deep penetration into the Soviet Union. The crews were trained in loft bombing, or toss bombing, as a high-risk method of bombing where the B-47s came in at a low level at 490 mph and then climbed abruptly near the target before releasing a nuclear weapon. Stress and fatigue incurred in low-altitude operational practices led to several wing failures and crashes, so an extensive refit program was begun in 1958 to strengthen the wing mountings.
In June 1960, the Strategic Air Command began testing a B-47 dispersal program in which the aircraft were operated out of civilian airfields in times of crisis. This dispersal was intended to shorten reaction time and ensure the survivability of the force. The frequency of these dispersals occurred during the Cuban Missile Crisis in 1962.
Phaseout of the B-47 began in 1961 with the disbandment of six bomb wings. This decision was executed by Secretary of Defense McNamara, placing a higher emphasis on strategic missile systems instead of the B-47 bombers. A deadline for complete B-47 phase-out had been established in June 1966, and the remaining five B-47 wings were phased out six months earlier during the last half of 1965. The last USAF operational aircraft, WB-47Es assigned to the Air Weather Service, were retired from the service in September 1969.
A total of 1,341 B-47Es were produced for USAF SAC; 691 were built by Boeing, 386 were built by Lockheed, and 264 were built by Douglas. Most B-47Bs, as mentioned earlier, were rebuilt to B-47E standards and given the designation of B-47B-II, though they were often called B-47Es.
