A slew of U.S. bomber aircraft have transcended the military community to become iconic weapons of war in the eyes of the general public. The stable of World War II bombers, for example, like the B-17 Flying Fortress, B-24 Liberator, and B-29 Superfortress, which are typically associated with the liberation of Europe and Asia.
Then you’ve got the B-52 Stratofortress, a massive eight-engine brute that has been in service for seven decades without any plans for retirement. But there were also machines that debuted in the small window between the iconic classes of U.S. bombers; after World War II but before the B-52 became a mainstay of U.S. aerial power. And generally, understandably, those bombers have been forgotten as relics of a transitional period in military technology.
But one such example is particularly important: the B-47 Stratojet, the U.S.’s first jet bomber.
Research and Development of the Early Jet Bomber
The B-47 Stratojet was introduced in 1951 as a long-range, high-altitude, jet-powered strategic bomber. The B-47 was designed to avoid enemy interceptor aircraft and deliver nuclear weapons deep within the Soviet Union’s territory.
In 1943, the U.S. Army Air Forces (USAAF) made an informal request for the design of a jet-powered reconnaissance bomber. Jet technology was sparkling new in 1943 and had not yet been applied to a bomber platform – but the merits of such an application were obvious. Several aerospace companies responded to the USAAF’s request, and began conducting research into jet bomber technology.
By 1944, the USAAF had formalized its request, asking for proposals for a jet bomber with impressive specifications: 550 mile per hour top speed; 450 mile per hour cruising speed; 3,500 mile range, and; 45,000 foot service ceiling. North American Aviation, Convair, Boeing, and the Glenn Martin Company each submitted proposals – and all four companies were awarded study contracts. NAA and Convair were asked to focus on four-engine designs – which would become the B-45 and XB-46 respectively. Boeing and Martin were asked to focus on six-engine designs – which would become the B-47 and XB-48 respectively.
Design Influence from German Aeronautics
In 1945, the Army Air Forces Scientific Advisory Board inspected a secret German aeronautics lab. Embedded with the inspection team was George S. Schairer, the chief of Boeing’s technical staff. During the inspection, the team found German airplane models incorporated something novel, something rumored but unconfirmed: swept wings. Extensive wind tunnel data confirmed that the swept wing design was quite effective. Urgently, Schairer wired back to Boeing.
“Stop the bomber design,” Schairer wrote. The new bomber, under development back home with Boeing, featured straight wings. But Schairer was now a convert; he wanted the new bomber to have swept wings.
Back home, Boeing redesigned its jet bomber to include wings and tail that swept back at 35 degrees. Further revisions were to follow, especially with respect to engine placement and landing gear placement.
By April 1946, Boeing was satisfied with all of its tweaks. The USAAF ordered two prototypes, which were designated as the XB-47. The XB-47s rolled off the assembly line, ready for flight testing, just a few days before the USAAF separated from the U.S. Army to become a distinct service, the U.S. Air Force. The newly formed USAF set to work, exhaustively testing the new XB-47.
A New Jet Bomber in the New Air Force
Because the new jet bomber incorporated so many cutting-edge technologies, the platform was heavily tested. Test pilot Robert Robbins, who helped debut the XB-47, was initially skeptical about the new bomber. “Oh God, please help me through the next two hours,” Robbins prayed before the XB-47’s first flight. Robbins’ fears were alleviated, however; the XB-47 could fly.
There were some problems, however. The prototype had a tendency to “Dutch roll,” or weave side to side in an uncontrolled yawing motion. A “yaw damper” control system was installed to prevent such yawing. The bomber also had a habit of pitching up at maximum speed – a dangerous flight characteristic caused stalling due to upward pitching of the outboard section of the wing. To remedy the upward pitching, Boeing installed “vortex generators” to prevent the airflow separation that led to upward pitching and stalling. And tragically, during an early test flight, pilot Scott Osler was killed when the XB-47’s canopy ripped off at high speed. The co-pilot managed to land the bomber safely – and Boeing redesigned the canopy.
Stratojet in Service
The exhaustive tweaking and testing paid off in dividends – although the finished B-47 Stratojet still had problems.
The finished B-47 Stratojet was regarded as the fastest bomber in the world. USAF Col. Walter Boyne described the new bomber as a “sleek, beautiful outcome that was highly advanced.” The B-47 was said to fly with a light touch, more like a fighter jet than a bomber. The B-47 was so aerodynamically clean that high-speed landing gear (180 knots) was required; the landing was assisted, however, with a ribbon-like drag chute that would slow the bomber down. And because of the hazards associated with such high-speed landings, the B-47 was the first mass-produced aircraft to be equipped with an anti-skid braking system (ABS).
Still, the B-47 was criticized for its high landing speed – which, when paired with the bomber’s sluggish takeoff performance made for a dangerous combination. The B-47 was also very particular about its landing attitude. If landed at the wrong attitude, the bomber would porpoise – and sometimes skid onto one wing before cartwheeling. Another serious problem: the wings flexed in flight, which had a tendency to affect flight control. Also, the B-47 was a “maintenance hog.”
Despite the drawbacks, over 2,000 B-47s were produced. The standard variant served proudly until 1969, while the electronic-intelligence variant, the EB-47, served until 1977.