Sparked by refugee physicists in the United States, the program was slowly organized after nuclear fission was discovered by German scientists in 1938, and many US scientists expressed the fear that Hitler would attempt to build a fission bomb.
In 1942 General Leslie Groves was chosen to lead the project, and he immediately purchased a site at Oak Ridge, Tennessee, for facilities to separate the necessary uranium-235 from the much more common uranium-238. He also appointed theoretical physicist J. Robert Oppenheimer as director of the weapons laboratory, built on an isolated mesa (flat land area) at Los Alamos, New Mexico.
After much difficulty an absorbent barrier suitable for separating isotopes of uranium was developed and installed in the Oak Ridge gaseous diffusion plant. Finally, in 1945, uranium-235 of bomb purity was shipped to Los Alamos, where it was fashioned into a gun-type weapon. In a barrel, one piece of uranium was fired at another, together forming a supercritical, explosive mass.
Another type of atomic bomb was also constructed using the synthetic element plutonium. Enrico Fermi built a reactor at Chicago in late 1942, the prototype of five production reactors erected at Hanford, Wash. These reactors manufactured plutonium by bombarding uranium-238 with neutrons. At Los Alamos the plutonium was surrounded with high explosives to compress it into a super dense, super critical mass far faster than could be done in a gun barrel. The result was tested at Alamogordo, New Mexico, on July 16, 1945, and was the first explosion of an atomic bomb.
Physicists from 1939 onward conducted much research to find answers to such questions as how many neutrons were emitted in each fission, which elements would not capture the neutrons but would moderate or reduce their velocity , and whether only the lighter and scarcer isotope of uranium (U-235) fissioned or the common isotope (U-238) could be used. They learned that each fission releases a few neutrons. A chain reaction, therefore, was theoretically possible, if not too many neutrons escaped from the mass or were captured by impurities. To create this chain reaction and turn it into a usable weapon was the ultimate goal of the Manhattan Project.
