quality:correction, structural:expansion
Origins

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 theoretically possible. There were fears that a German atomic bomb project would develop one first, especially among scientists who were refugees from Nazi Germany and other fascist countries. In August 1939, Hungarian-born physicists 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 of the National Bureau of Standards 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 Advisory Committee on Uranium became the National Defense Research Committee (NDRC) on Uranium when that organization was formed on 27 June 1940.

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. The NDRC Committee on Uranium became the S-1 Section of the OSRD; the word "uranium" was dropped for security reasons. 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.

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 the atomic program after he convened a meeting with Vannevar Bush and Vice President Henry A. Wallace. He created a Top Policy Group consisting of himself—although he never attended a meeting—Wallace, 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.