The Boeing B-47 jet bomber was a major postwar innovation in combat jet design, and led to the development of modern jetliners. While it never saw serious combat use, it was the mainstay of US strategic defense in the 1950s. This document provides a short history of the B-47.

ORIGINS:

* The B-47 arose from a 1943 US Army Air Force (USAAF) requirement for a jet bomber / reconnaissance aircraft, which evolved into a formal request the next year. The request specified a speed of 805 KPH (500 MPH) or more, a range of 5,635 kilometers (3,500 miles), a service ceiling of 12.2 kilometers (40,000 feet), and envisioned use of the General Electric TG-180 turbojet engine, then in development.

North American, Convair, and Boeing submitted proposals. The first Boeing proposal, the "Model 424", was a modification of a conventional propeller-driven bomber design, basically a scaled-down version of the Boeing B-29 Superfortress fitted with four TG-180 turbojets.

The US National Advisory Committee for Aeronautics (NACA), the ancestor of the modern US National Aviation & Space Administration or (NASA) performed wind tunnel tests on a model of the design, or more specifically a composite of the designs submitted by the manufacturers, as the three submissions were generally similar.

By this time, the war in Europe was obviously winding to a close. General "Hap" Arnold, head of the USAAF, asked the prestigious expatriate Hungarian aerodynamicist Theodore von Karman, of the California Institute of Technology, to form up a committee of American scientists to go to Europe and examine captured German technology.

The result was the "Scientific Advisory Group". One of the members was Boeing's chief aerodynamicist, George Schairer. During his visit to Germany, Schairer examined data obtained by German aircraft manufacturers on the advantages of swept wings, and became so convinced of the merits of such a design that in May 1945 he wrote a letter to Boeing management suggesting the matter be investigated.

* Meanwhile, the USAAF had awarded study contracts to all three aircraft manufacturers working on the jet bomber project, as well as to the Martin company, which had also decided to join the competition.

The NACA wind tunnel tests showed that the model suffered from excessive drag. Boeing engineers then tried a revised design, the "Model 432", that had the four engines buried in the forward fuselage, but though the Model 432 had some structural advantages, changing the engine layout didn't really reduce drag all that much. The Boeing engineers turned to the swept-wing data obtained from the Germans and promoted by Schairer. A little design work by Boeing aerodynamicist Vic Ganzer led to an optimum sweepback of 35 degrees.

Boeing then modified the Model 432 design with swept wings and tail, resulting in the "Model 448", which was presented to the USAAF in September 1947. The Model 448 had the four TG-180s in the forward fuselage as had the Model 432, plus two TG-180s buried in the rear fuselage. The Boeing project manager, George Martin, had decided that the company's entry into the bomber competition needed greater range and performance, and that led to six engines rather than four.

Boeing submitted the Model 448 to the USAAF in October 1945, only to have it rejected immediately. The Air Force strongly disliked fitting the engines in the fuselage, since that made engine fire or disintegration catastrophic. The engines would have to be moved back out on the wings.

That led straight back to the drag problem, but the engineering team came up with a clean, elegant solution, with the engines in streamlined pods attached to the wings. This innovation led to the next iteration, the "Model 450", which featured two TG-180s in a single pod mounted on a pylon about a third of the way outboard on each wing, plus another engine slung from the wingtip.

* The Air Force liked the new configuration, and so the Boeing team continued to refine it. One problem was landing gear. There was no space for landing gear in the thin wings, and trying to put conventional tricycle landing gear in the fuselage would have ruined the aircraft's streamlining and degraded its performance. Furthermore, the USAAF was now also insisting that the bomber be able to carry an atomic bomb. As such weapons were very big at the time, that meant a long bombbay, further limiting space for landing gear.

The solution was a "bicycle" landing gear configuration, with the two main gear assemblies arranged in a tandem, not a side by side, configuration. Outrigger landing gear was to be fitted to the inboard engine pods. The concept had already been tested on a modified Martin B-26 Marauder aircraft, the "Middle River Stump Jumper", named after the Martin plant at Middle River, Maryland.

However, the bicycle landing gear made it difficult for a pilot to "rotate" an aircraft into a nose-up position for takeoff. Again, the solution was simple: the landing gear was designed so that the nose-up position was the default. This little change would have a very pleasing effect on an aircraft that was already shaping up to be very elegant, giving the machine the appearance of being ready to leap into the air even when it was sitting still.

There were some other tweaks to the design, such as a wingtip extension to improve range. This had the effect of moving the outboard engines from a wingtip position to an underwing position towards the end of the wings.

* The USAAF was very pleased with the refined Model 450 design, and in April 1946 the service ordered two prototypes, to be designated "XB-47". Assembly began in June 1946. People involved with the project were very excited, since they believed, correctly as it turned out, they were working on a breakthrough in aircraft design.

However, there was a widespread disinterest in the machine through the rest of the Boeing company, it seems partly because it was so futuristic, leading many to dismiss it as a whizzy experimental aircraft that would be impractical for operational use. Pictures of the initial rollout of the first XB-47 prototype show only about a hundred people watching.

The XB-47 prototype first flew on 17 December 1947, with test pilots Robert Robbins and Scott Osler at the controls. The aircraft flew from Boeing Field in Seattle to the Moses Lake Airfield in central Washington state, in a flight that lasted 52 minutes. There were no major problems, except that Robbins had to pull up the flaps with the emergency hydraulic system and the engine fire warning lights kept popping on, the sensor technology being very unreliable at the time. Robbins reported that the flight characteristics of the aircraft were good.