A Manhattan office name hid the factories that built the bomb
The Manhattan Project, a U.S.-led World War II effort with Britain and Canada, built the first nuclear weapons, peaking near 130,000 workers and almost $2 billion (about $28 billion today). General Leslie Groves directed from 1942–46; J. Robert Oppenheimer ran Los Alamos bomb design.
The physics came first. Otto Hahn and Fritz Strassmann found nuclear fission in 1938, and Lise Meitner and Otto Frisch explained it, making a uranium bomb conceivable and raising fears that Germany would build one. In August 1939 Leo Szilard and Eugene Wigner drafted a warning letter, got Albert Einstein to sign it and sent it to President Roosevelt, who set up Lyman Briggs's Advisory Committee on Uranium. In early 1940 the Navy gave Columbia University $6,000, most of which Enrico Fermi and Szilard spent on graphite.
Britain was moving faster. At Birmingham, Frisch and Rudolf Peierls calculated in 1940 that the critical mass of uranium-235 was around 10 kilograms, light enough for a bomber, and the MAUD Committee backed building a weapon. When the Australian physicist Mark Oliphant visited in August 1941, he found that Briggs had locked the British report in a safe; Oliphant's lobbying won over Ernest Lawrence. On 9 October 1941 Roosevelt approved speeding up the programme and gave it to the Army, which had more experience running big construction jobs.
The money went mostly into industry: more than four-fifths of the budget built and ran plants making fissile material, uranium enriched at Clinton in Tennessee and plutonium bred at Hanford in Washington, in the first industrial-scale reactors. The implosion design, first treated as a complicated fallback, became essential in 1944 once Hanford plutonium proved to contain too much plutonium-240 for a gun-type weapon. An implosion device was the first nuclear explosion ever, at the Trinity test in New Mexico on 16 July 1945.
After the war the organisation ran the Operation Crossroads tests at Bikini Atoll, fostered national laboratories and radiology research, and laid groundwork for a nuclear navy. It kept control of American atomic work until the Atomic Energy Commission took over in January 1947.
Source: Manhattan Project
