“At the secret desert lab, scientists had devised two atomic bomb designs: an implosion type using plutonium, and a gun type using uranium. Parsons’s primary assignment was the assembly of the gun-type uranium bomb. He would actually complete that job inside the belly of the Enola Gay, the B-29 Superfortress that would deliver the bomb to its target. Because B-29s had a proclivity for crashing on takeoff, and because the uranium bomb was so dangerous, Groves decided that the “gadget,” as they called it, must be assembled in the air.”