Thousands of people were involved in the making of the atomic bombs. By 1945, the Manhattan Project had nearly 40 laboratories and factories and employed about 200,000 people. Some of the more known physicists involved were Albert Einstein, Enrico Fermi, Richard Feynman, J. Robert Openheimer, and Harold Urey.
Physicist, Enricho Fermi, successfully controlled a nuclear reaction in his own reactor which was called Chicago Pile-1 or CP-1. This was in 1942.
Albert Einstein had sent a letter to president Roosevelt informing him about the Germans and the use of nuclear weapons. With Einstein help, the United States got ahead in the race for the first nuclear weapon.