Everyone involved was confident the gun-type bomb would work. They'd tested bringing two pieces of enriched uranium close together in criticality experiments. That experiment killed several people at Los Alamos.

The big worry was it going off prematurely. When the Enola Gay took off to drop the uranium bomb on Hiroshima, their orders were that if they had to turn back, dump the bomb in deep water. A billion dollars to the bottom of the sea. That was preferable to taking the chance of blowing the airfield on Tinian off the map if the B-29 had a fire on landing.

Making enriched uranium was slow, but working. The gaseous-diffusion plant cascades were working, after months of startup. The second gaseous diffusion plant at Oak Ridge, K-27, opened in September 1945, a few weeks after the end of the war. If the war had dragged on, there would have been more uranium bombs.