In April, 1943, Los Alamos was opened. The work at Los Alamos centered around the development of a delivery system, and the two methods considered were the gun method and the implosion method. The gun method called for two non-critical elements to be fired at each other, contact turning them critical, but it was rejected as being to large for aerial delivery. Implosion called for the collapse of non-critical wedges, which would become critical when brought into contact, and it was the method that was eventually used. In February, 1944, the first slightly enriched Uranium (13% U-235) arrived at Los Alamos.
In September, 1944, 63% Uranium 235 arrived at Los Alamos, and the designs for Fat Man and Little Boy were finalized. Fat Man, composed of Plutonium shell within an array of shape charges wrapped around it, had a diameter of 60 inches, a length of 128 inches, and a weight of 10,000 pounds. Little Boy, composed of a long cannon with a Uranium (235) bullet, and U-235 target rings in the muzzle, had a diameter of 28 inches, and a length of 120 inches, and a weight of 9000 pounds.
On December 17th, 1944, the 509th composite bomber group, composed of specially modified B-29 Superfortresses designed to carry atomic bombs, was formed to deliver the bomb to targets in Japan.
In March, 1945, the S-50 plant at Oak Ridge came fully online, and began to supply Los Alamos with 85% enriched U-235.
On July 16th, 1945, the first nuclear bomb test occurred at the Trinity test site in New Mexico. It was the Fat Man design, and it was successful.