After abandoning the plutonium gun, Oppenheimer streamlined the uranium gun program. The remaining work, all experimental and straight-forward, was consolidated in one ordnance group under Lt. Comdr. A. Francis Birch. Birch's group completed and tested the uranium gadget design by February 1945. Meanwhile, metallurgists decided that 235U was strong enough to be used in its natural state, so that alloy research could be discontinued. They developed suitable large-scale metal reduction and fabrication techniques. Polonium production for the initiators was on schedule.

As 235U production slowly increased and the laboratory made arrangements for fabricating gun parts, a quiet but dramatic change was occurring in the gun program. Before March 1945, this program had centered on perfecting a reliable method of assembling 235U. Now Birch had to turn the gadget into a bomb that could be delivered by an airplane. The bomb had to contain the gun gadget, offer protection from antiaircraft fire, house all the components (including antenna, fuzing, and circuitry), and provide a reliable, stable flight. The design of the gadget could still be changed slightly, after being frozen in February. But once the drop tests began, extensive changes could not be made without jeopardizing Groves's July deadline.

Conversion to the Uranium Gun

With the cancellation of the plutonium gun in July 1944, the need to develop a multifaceted high-velocity gun suddenly vanished. Fifteen months of intensive effort had gone into the physical design of the plutonium gun's components, into interior ballistics, and the mechanical properties of plutonium.