Birth of a Turbojet Bomber
In 1943, before any turbojet aircraft had entered service, the Army Air Force issued requirements for a turbojet-powered bomber. (The turbojet engine is the basic engine of the jet age. It works by compressing air, forcing it into a combuster, which sprays fuel on it and ignites it. The air burns continuously like a blow torch, through the turbine, which extracts energy to work the compressor, and out the back to thrust the aircraft forward.)
Boeing responded by redesigning its most advanced extant airplane, the B-29 Superfortress, with turbojet engines. This approach proved unfeasible, and eventually a new straight wing design with on-wing engine nacelles (enclosures) emerged with reasonable performance expectations. Not satisfied that Boeing had a competitive bomber design, engineer George Schairer, who in 1945 was in Germany examining sweptback wing research data, suggested using such a wing.
Looking into the future, in 1943 Boeing also began to study and design gas turbine engines (that is, jet engines), which it would build and market beginning in 1946.
Concerned about the short range of operational jet aircraft, the Air Force also funded development of several new piston engine bombers. Boeing’s B-50 Superfortress, a more powerful, faster enhancement of the B-29, flew six months before the B-47, on June 25, 1947. The two bombers were produced concurrently; 372 B-50s were built.
Boeing’s first jet airplane would be a technological blockbuster, not a piston engine design with turbojets hung on. The 35° swept back wing was chosen to attain high speed, as much as 100 mph faster than competing designs. Its advantages were known and tested in the U.S. (and elsewhere) during World War II by the National Advisory Committee on Aeronautics (NACA). Nor was the B-47 Boeing’s first swept wing design. Way back in 1935, the model 306 fighter design (before the 307 Stratoliner airliner) featured a 30° swept wing, tricycle landing gear, and a pusher propeller.
Sweptback wings date to the first decade of the powered airplane. Britain’s John Dunne flew a sweptback wing biplane with inherent stability (the reason he used swept wings) in 1910. The U.S. Army and Navy evaluated his later swept wing designs, as armed warplanes in 1914-1915; the first military services to do so. Interestingly, their 32° wing sweep was nearly the same as the B-47. In the late 1930s, U.S. light planes with 25° sweptback wings were in production.
Under Wing Pylon Mounted Engines
With the wing design settled, the next issue was where to mount the engines. Fuselage and on-wing positioned engines had aerodynamic drag and safety problems. Engineers hit upon the idea of using under wing pylon mounted engines, which gave a low drag, aerodynamically clean wing, and being distanced from the airframe, were safer in case of fire. The final design paired two engines near the fuselage (the central body of the aircraft) and another close to the tip on each wing.
This time the Russians were first. Ilyushin’s IL-22 bomber beat the Stratojet into the air by five months, flying on July 24, 1947. A small, straight winged, slow airplane, the Il-22’s four turbojet engines were individually mounted on pylons below the wing. Only one bomber was built; it crashed two months later and was lost. Douglas added two turbojets on pylons beneath the straight wings of its experimental piston-engined XB-42A Mixmaster bomber, which flew even before the Ilyushin, on May 24, 1947.
A Fighter-Like Bomber
Accentuating the Stratojet’s long, streamlined fuselage, perched near the forward end, was an elongated bubble type canopy that was usually associated with considerably smaller fighters. Under its pressurized Plexiglas cover sat the pilot followed by the co-pilot/tail gunner, who were joined by the navigator/bombardier below in the forward fuselage. In contrast to this crew of three, the shorter and much lighter B-29 had an 11-member crew.
Bicycle type landing gear with fuselage mounted paired wheels fore and aft, gave the B-47 its pronounced nose up attitude while on the ground. Two small outrigger wheels were mounted below the inboard engine nacelles for lateral balance.
Motive Force: the J-47 Turbojet
In the last year of World War II, the highest-flying jet airplane over Europe with the most powerful engine was the U.S. Lockheed YP-80A Shooting Star fighter. General Electric’s J-33 turbojet engine powering the YP-80A, produced 4,000 lb. of thrust, twice that of any other operational engine. General Electric (GE) had entered the aircraft gas turbine field on July 7, 1941, prior to the Pearl Harbor attack, and before England’s Frank Whittle graciously licensed GE to develop his pioneer jet engine further. By the end of the war, in addition to the J-33, GE had produced the most powerful, by far, axial flow (the dominant type today) turbojet running, and a turboprop engine. (The turboprop engine uses the thrust of hot gasses to turn a propeller.)
Germany pioneered the use of the axial flow jet engine, building several thousand that powered the first operational jet fighters and bombers used in combat. (Axial flow engines have rotating compressors that push air along the axis of rotation. In centrifugal flow compressors, air enters in the center and is spun to the outside.)
Germany's axial flow engines were heavy, unreliable and had a major fault -- they did not function well above 30,000 feet altitude. The Shooting Star had a nearly 15,000-foot altitude advantage over its German adversaries, a serious advantage in combat.
Great Britain flew an axial flow jet engine powered airplane during the war, as did the U.S. and Japan, while Russia ground tested engines. Britain went on to great postwar success by manufacturing centrifugal flow (like the earlier J-33) engines, while the U.S. took up the gauntlet and developed the first truly successful axial flow jet engines. GE further refined its wartime J-35 into the J-47 turbojet engine, which was capable of problem free flight to nearly 50,000 feet altitude.
Six J-47s powered production B-47s to high speed and stratospheric altitude. It was the engine of choice for numerous airplanes. In the North American Aviation F-86 Sabre fighter, the J-47 powered it to supersonic speed in a slight dive -- the second successful supersonic airplane and the first in production. During Korean War combat against the centrifugal flow jet engine powered Mig-15 fighter, the Sabre prevailed mightily. More than 35,000 J-47s were built, probably the most of any turbojet engine.
The Stratojet even with six jet engines was underpowered during certain takeoff conditions; consequently it also had rocket engines. Eighteen, later 33, solid fuel rocket engines were mounted in or under the lower aft fuselage. The rocket engines gave the B-47 a spectacularly steep climb angle, and produced a vast smoke cloud and horrendous noise.
The initial B-47B production version entered squadron service during 1952. Its high speed convinced the Air Force that only radar controlled twin tail guns were needed for protection. Two jet bombers preceded the Stratojet, including the combat-proven World War II German Arado Ar 234 Lightning. Boeing’s B-47 would serve the Air Force long and well. At peak usage the Strategic Air Command had more than 1,500 Stratojets on strength in its bomber wings. As the first successful, large 600+ mph jet aircraft, the B-47 was followed into service three years later by the similarly capable Soviet Tupolev Tu-16 Badger (a twin jet with three gun turrets; a development of the Tu-4, the B-29 replica) and the British Vickers Valiant B.1 (four jet engines, unarmed). That same year, 1955, the Stratojets’ successor, the eight-turbojet engine Boeing B-52B Stratofortress heavy bomber entered service.
Propellers turned by powerful turboprop engines replaced the inboard pair of engines on two test B-47Ds. The stated purpose of the conversion was to test the new engines; the Air Force had long since abandoned the turboprop engine as a means to extend a bomber’s range. Interestingly, the B-47D was nearly as fast as the normal bomber, at 597 mph. At the time, in 1955, the Soviet Tu-20 Bear bomber became known to the West -- it had swept back wings and four turboprop engines. Several versions of the 540 mph Bear remain in Russian service in 2002.
Time line comparison: both the B-47 and F-86 Sabre began as straight wing designs during World War II, changed to 35° swept back wings, were powered by the J-47, and flew in 1947. The Sabre took flight first, on October 1, a meager 2.5 months before the Stratojet. Early in the B-47’s career, the F-86 was one of only two airplanes that could keep pace (the Mig-15 was the other, it also flew in 1947, on December 30). Being considerably less complex than the Stratojet, the Sabre entered Air Force squadron service three years earlier.
Many Roles, no CombatIn addition to serving as a bomber, the Air Force also procured specialized B-47s for photo-reconnaissance, Elint (electronic intelligence/eavesdropping) and weather reconnaissance (a WB-47E is displayed at the Seattle Museum of Flight) missions. Although the Stratojet entered service during the Korean War, it did not join the B-29s, B-50s, and B-17s used in that conflict. The B-47, in fact, never attacked a foe or dropped a bomb in anger in its career. No foreign air forces operated the Stratojet, but the Navy used three for test purposes.
During the course of many Elint missions flown near (and over?) Iron Curtain countries, two RB-47Hs were attacked by Communist fighters. In July 1960, shortly after the U-2 incident in May (in which an American spy plane, the U-2, piloted by Francis Gary Powers, was brought down in the Soviet Union), a RB-47H flying near Murmansk was shot down. Four of its six crewmen were killed and the two survivors were captured and imprisoned in Moscow’s Lubyanka prison for seven months before being released. In 1965, North Korean Mig-17s attacked a second RB-47H off the coast of Korea. Cannon fire from the Stratojet’s tail guns drove off the fighters, and the damaged bomber was able to land in Japan, without any injury to crewmen.
During the course of many Elint missions flown near (and over?) Iron Curtain countries, two RB-47Hs were attacked by Communist fighters. In 1960, shortly after the U-2 incident in May (in which an American spy plane, the U-2, piloted by Francis Gary Powers, was brought down in the Soviet Union), a RB-47H flying near Murmansk was shot down, and its six crewmen lost. In 1965, North Korean Mig-17s attacked a second RB-47H off the coast of Korea. Cannon fire from the Stratojet’s tail guns drove off the fighters, and the damaged bomber was able to land in Japan, without any injury to crewmen.
Finis. On October 31, 1969, the B-47 ended its long, 17-year Air Force career. A weather reconnaissance WB-47E flew to Davis-Monthan Air Force Base, Arizona, for storage.
Misguided, Sincere Flattery
Late into the 1950s, an East German company attempted to enter the jet airliner business by emulating the Stratojets’ features closely, and as it transpired, far too closely. The four engine VEB BB-152 was to carry 72 passengers for 1,500 miles at 500 mph, with a crew of three. It featured a high sweptback wing with under wing pylon mounted paired turbojets, and a bicycle type landing gear with outrigger wheels. Even the B-47’s front fuselage windows were faithfully included. All in all, a very atypical airliner design. The sole prototype crashed shortly after flying on December 4, 1958, which ended the program.
The two XB-47 prototypes were the last airplanes to be manufactured in Boeing's original Plant 1; production aircraft were built in Boeing’s Wichita, Kansas, facility. During the Korean War the Air Force wanted Stratojets faster than Boeing could build them, therefore a shared production program similar to that of the B-17 and B-29 was implemented. Douglas and Lockheed built B-47s under license in government factories. Of the 2,040 bombers constructed, Boeing assembled 1,373, 67 percent of the total. More Stratojets were built than any other jet bomber.
The second XB-47 is displayed at the Octave Chanute Aerospace Museum in Rantoul, Illinois.