The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. The Manhattan Project employed nearly 130,000 people at its peak and cost nearly US$2 billion (equivalent to about $28 billion in 2024).
From 1942 to 1946, the project was directed by Major General Leslie Groves of the U.S. Army Corps of Engineers. Nuclear physicist J. Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the bombs. The Army program was designated the Manhattan District, as its first headquarters were in Manhattan; the name gradually superseded the official codename, Development of Substitute Materials, for the entire project. The project absorbed its earlier British counterpart, Tube Alloys.
Over 80 percent of project cost was for building and operating the fissile material production plants. The project proposed both highly enriched uranium and plutonium as fuel for nuclear weapons. Enriched uranium was produced at the Clinton Engineer Works in Tennessee. Plutonium was produced in the world's first industrial-scale nuclear reactors at the Hanford Engineer Works in Washington. These sites were supported by dozens of other facilities across Canada, the US and the UK.
The weapons designs were produced at the Los Alamos Laboratory in New Mexico, and resulted in two weapon designs used during the war: Little Boy (enriched uranium gun-type) and Fat Man (plutonium implosion). The Fat Man design was initially a low priority fallback option, as it was complex and required explosive lenses, but in 1944 it was confirmed that plutonium from Hanford was not suitable for a gun-type bomb because of the high proportion of plutonium-240. The first nuclear device ever detonated was an implosion-type bomb during the Trinity test, conducted at White Sands Proving Ground in New Mexico on 16 July 1945. The project was responsible for developing the specific means of delivering the weapons onto military targets, and for the use of the Little Boy and Fat Man bombs in the atomic bombings of Hiroshima and Nagasaki in August 1945.
The project was also charged with gathering intelligence on the German nuclear weapon project. Through Operation Alsos, Manhattan Project personnel served in Europe, sometimes behind enemy lines, where they gathered nuclear materials and documents and rounded up German scientists. Despite the Manhattan Project's own emphasis on security, Soviet atomic spies penetrated the program.
In the immediate postwar years, the Manhattan Project conducted weapons testing at Bikini Atoll as part of Operation Crossroads, developed new weapons, promoted the development of the network of national laboratories, supported medical research into radiology, and laid the foundations for the nuclear navy. It maintained control over American atomic weapons research and production until the formation of the United States Atomic Energy Commission (AEC) in January 1947.
The discovery of nuclear fission by Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation by Lise Meitner and Otto Frisch, made an atomic bomb using uranium theoretically possible. There were fears that a German atomic bomb project would develop one first. In August 1939, Leo Szilard and Eugene Wigner drafted the Einstein–Szilard letter, which warned of the potential development of "extremely powerful bombs of a new type". It urged the United States to acquire stockpiles of uranium ore and accelerate the research of Enrico Fermi and others into nuclear chain reactions. They had it signed by Albert Einstein and delivered to President Franklin D. Roosevelt.
Roosevelt called on Lyman Briggs to head an Advisory Committee on Uranium; Briggs met with Szilard, Wigner and Edward Teller in October 1939. The committee reported back to Roosevelt in November that uranium "would provide a possible source of bombs with a destructiveness vastly greater than anything now known." In February 1940, the U.S. Navy awarded Columbia University $6,000, most of which Fermi and Szilard spent on graphite. A team of Columbia professors including Fermi, Szilard, Eugene T. Booth and John Dunning created the first nuclear fission reaction in the Americas, verifying the work of Hahn and Strassmann. The same team subsequently built a series of prototype nuclear reactors (or "piles" as Fermi called them) in Pupin Hall at Columbia but were not yet able to achieve a chain reaction.
The work of Briggs' committee was seen as insufficiently urgent by some of the scientists who were familiar with it. Karl T. Compton complained in early 1941:
As I analyze the situation, Briggs, who is by nature slow, conservative, methodical and accustomed to operate at peace-time government bureau tempo, has been following a policy consistent with these qualities and still further inhibited by the requirement of secrecy. . . . Considered as an element of the present war emergency, speed in attaining the objective is certainly more important than excessive secrecy, as would be abundantly evident if the German scientists should actually get some of the applications into use.
When the National Defense Research Committee (NDRC) was formed on 27 June 1940 to coordinate scientific research, the Advisory Committee on Uranium was absorbed into the organization. On 28 June 1941, Roosevelt signed Executive Order 8807, which created the Office of Scientific Research and Development (OSRD), under director Vannevar Bush. The office was empowered to engage in research and large engineering projects, and was immediately converted into the OSRD Section on Uranium. In July 1941, Briggs proposed spending $167,000 on researching uranium, particularly the uranium-235 isotope, and plutonium, which had been isolated for the first time at the University of California in February 1941. The work, however, was still being conducted at a relatively small scale and scope, as Bush and other administrators were still skeptical that the work could achieve quick success.
In Britain, Frisch and Rudolf Peierls at the University of Birmingham had made a breakthrough investigating the critical mass of uranium-235 in June 1939. Their calculations indicated that it was within an order of magnitude of 10 kilograms (22 lb), small enough to be carried by contemporary bombers. Their March 1940 Frisch–Peierls memorandum initiated the British atomic bomb project and its MAUD Committee, which unanimously recommended pursuing the development of an atomic bomb. In July 1940, Britain had offered to give the United States access to its research, and the Tizard Mission's John Cockcroft briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as advanced.
As part of the scientific exchange, the MAUD Committee's findings were conveyed to the United States. One of its members, the Australian physicist Mark Oliphant, flew to the US in late August 1941 and discovered that data provided by the MAUD Committee had not reached key American physicists. Oliphant set out to find out why the committee's findings were apparently being ignored. He met with the Uranium Committee and visited Berkeley, California, where he spoke persuasively to Ernest O. Lawrence. Lawrence was sufficiently impressed to commence his own research into uranium. He in turn spoke to James B. Conant, Arthur H. Compton and George B. Pegram. Oliphant's mission was therefore a success; key American physicists were now aware of the potential power of an atomic bomb.
On 9 October 1941, President Roosevelt approved accelerating the atomic program. He created a Top Policy Group consisting of himself—although he never attended a meeting—Vice President Henry A. Wallace, Vannevar Bush, Conant, Secretary of War Henry L. Stimson, and the Chief of Staff of the Army, General George C. Marshall. Roosevelt chose the Army to run the project rather than the Navy, because the Army had more experience with managing large-scale construction. He agreed to coordinate the effort with the British and on 11 October sent a message to Prime Minister Winston Churchill, suggesting that they correspond on atomic matters. Bush had, by this point, already begun expanding and altering the leadership of the OSRD Section on Uranium, retaining Briggs as chairman but adding and elevating other personnel. The section itself was renamed to Section S-1.