Initial Phase
The project was initiated in October 1943 when Dr. Norman F. Ramsey, a member of the Los Alamos National Laboratory Group E-7, identified the B-29 Superfortress as the only airplane in the U.S. inventory capable of carrying either type of the proposed weapons shapes: the tubular "gun-type fission weapon" shape (Little Boy) and the oval plutonium implosion weapon shape (Fat Man). Furthermore, because the attachment box for the main wing spars was located between the bomb bays on the B-29, the gun-type weapon could only be a maximum of 2 ft (0.6 m) in diameter. Prior to the decision to use the B-29 serious consideration had been given to using the British Avro Lancaster to deliver the weapon, which would have required much less modification, but Major General Leslie R. Groves, Jr., the commander of the Manhattan Project, and the Chief of United States Army Air Forces, General Henry H. Arnold wished to use an American plane, if this was possible.
USAAF sent instructions to its Army Air Forces Materiel Command at Wright Field, Ohio, on November 30, 1943, for a highly-classified B-29 modification project. The Manhattan Project would deliver full-sized mockups of the weapons shapes to Wright Field by mid-December, where AAFMC would modify an aircraft and deliver it for use in bomb flight testing at Muroc Army Air Field, California.
B-29-5-BW 42-6259 (referred to as the "Pullman airplane" from an internal code name assigned it by the Engineering Division of AAF Materiel Command) was delivered to the 468th Bombardment Group at Smoky Hill AAB, Kansas on November 30, 1943, and flown to Wright Field, Ohio, on December 2. Modifications to the bomb bays were extensive and time-consuming. Its four 12 ft (3.7 m) bomb bay doors and the fuselage section between the bays were removed and a single 33 ft (10 m) bomb bay configured (the length of the gun-type shape was approximately 17 ft (5.2 m)). New bomb suspensions and bracing were attached for both shape types, with the gun-type suspension anchored in the aft bomb bay (although its length protruded into the forward bay) and the implosion type mounted in the forward bay. Separate twin-release mechanisms were mounted in each bay, using modified glider tow-cable attach-and-release mechanisms. To document the tests, motion picture camera mounts were installed in the rear bay.
The modifications were made using the dummy bomb shapes as models, and the gun-type shape (code-named Thin Man) proved to be a very close fit. All modifications were made by hand and the process required more than 6000 man-hours of labour which could not be completed until February. Engine problems systemic to the B-29 delayed delivery of the Pullman B-29 for flight testing until February 20, 1944.
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