23 January 2010
Less than six months separated the first flight of the Boeing 377 Stratocruiser and the first flight of the Boeing B-47 Stratojet in December of 1947. While the Stratocruiser represented in many ways the ultimate development of the B-29 Superfortress, there was a design evolution that connects the B-29 and the B-47 as well.
The genesis of the B-47 came in the midst of the Second World War when the US Army Air Forces (the USAF wasn't an independent branch until 1947) looked to the future and knew that jets were the way to go and decided a jet-powered bomber based on their top of the line aircraft, the B-29 Superfortress was what was needed next. Boeing's design team began with what was essentially a jet-powered development of the B-29 but ran into difficulties meeting the range and performance levels that the USAF desired. When the war ended in August 1945, Boeing's chief aerodynamicist, George Schairer, was already in Germany with a USAAF technical team that was evaluating captured German aeronautical research.
At the Luftwaffe research center at Volkenrode, the technical team was reviewing wind-tunnel research into swept wings. In a dry well Schairer and the team found hastily-dumped papers that showed the performance leap possible by combining jet engines and swept wings.
Schairer wrote a seven page later to the engineering team at Boeing working on what would become the B-47 Stratojet. The design process at the time had a design that had fuselage mounted engines and a straight tapered wing and Schairer detailed in his letter what he had found at Volkenrode and his thoughts at how it might benefit the jet bomber design. As a result, the design was reworked to feature a 35-degress swept wing- at the time designated the Boeing Model 448, it was the first of two major technological breakthroughs in the design of the B-47. At this point the design still had its engines mounted in the fuselage and team found that the thin, swept wing could bend excessively in some flight regimes.
The second technological breakthrough was to relocate the jet engines in pods in the wings. The weight of the engines offset bending in the wings and resulted in a more aerodynamic fuselage. It would set the pattern for all large jet aircraft from then onward.
When the XB-47 made its first flight in December 1947, it was only 44 years to the day of the Wright Brothers' first flight at Kitty Hawk. But there was one other aircraft that beat the XB-47 into the air as the first with swept wings, and that was the XP-86 Sabre from North American Aviation (later redesignated F-86). North American also had representatives on the same technical team in Germany with George Schairer.
Source: 747: Creating the World's First Jumbo Jet and Other Adventures from a Life in Aviation by Joe Sutter with Jay Spenser. Smithsonian Books, 2006, p53-57.