Editing Boeing B-47 Stratojet (section)

===Swept wings===
In May 1945, the [[United States Air Force Scientific Advisory Board|von Kármán mission of the Army Air Forces]] inspected the secret German aeronautics laboratory near [[Braunschweig]]. Von Kármán's team included the chief of the technical staff at Boeing, [[George S. Schairer]]. He had heard about the controversial swept-wing theory of [[Robert Thomas Jones (engineer)|R. T. Jones]] at Langley, but seeing German models of swept-wing aircraft and extensive supersonic wind-tunnel data, the concept was decisively confirmed. He wired his home office: "Stop the bomber design" and changed the wing design.<ref>Von Karman,'' Aerodynamics: Selected Topics in the Light of their Historical Developments'', 1954.</ref><ref name = "gungil 3940">Gunston and Gilchrist 1993, pp. 39–40.</ref> Analysis by Boeing engineer Vic Ganzer suggested an optimum sweepback angle of about 35 degrees.<ref>Cook 1991, p. 152.</ref> Boeing's aeronautical engineers modified the Model 432 with swept wings and [[empennage|tail]] to produce the "Model 448", which was presented to the USAAF in September 1945. It retained the four TG-180 jet engines in its forward fuselage, with two more TG-180s in the rear fuselage. The flush-mounted air intakes for the rear engines were inadequate while the USAAF considered the engine installation within the fuselage to be a fire hazard.<ref name="Peacock AE p33"/><ref name="Knaack p102">Knaack 1988, p. 102.</ref><ref name = "gungil 40">Gunston and Gilchrist 1993, p. 40.</ref>

The engines were moved to streamlined pylon-mounted pods under the wings, leading to the next iteration, the ''Model 450'', which featured two TG-180s in a twin pod mounted on a pylon about a third of the way outboard on each wing, plus another engine at each wingtip.<ref name = "gungil 40"/> The Army Air Force liked this new configuration, so Boeing's engineers refined it, moving the outer engines further inboard to about {{frac|3|4}} of the [[wingspan]]. The thin wings provided no space for tricycle main gear to retract so it would have needed a considerable bulge in the fuselage aft of the bomb bay for lateral stability. The only way to get a bomb-bay long enough for an A-bomb was to use a "bicycle landing gear",<ref>Cook 1991, p. 164.</ref> the two main gear assemblies arranged in a tandem configuration and outrigger struts fitted to the inboard engine pods. As the landing gear arrangement made [[Rotation (aviation)|rotation]] impossible, it was designed so that the aircraft rested on the ground at the proper angle for takeoff.<ref name="Peacock AE p33"/><ref name="Bowers Boeing p383">Bowers 1989, p. 383.</ref><ref name = "gungil 42">Gunston and Gilchrist 1993, p. 42.</ref>

Pleased with the refined Model 450 design, in April 1946, the USAAF ordered two prototypes, to be designated "XB-47".<ref name="Knaack p102-3">Knaack 1988, pp. 102–103.</ref> Assembly began in June 1947. The first XB-47 was rolled out on 12 September 1947,<ref name="Bowers Boeing p383"/> a few days before the USAAF became a separate service, the [[United States Air Force]] (USAF), on 18 September 1947. According to aviation authors Bill Gunston and Peter Gilchrist, Boeing subjected the first prototype to "one of the most comprehensive ground-test programmes ever undertaken".<ref name = "gungil 43"/>