The discovery of
nuclear fission by German chemists
Otto Hahn and
Fritz Strassmann in 1938, and its theoretical explanation by
Lise Meitner and
Otto Frisch, made the development of an
atomic bomb a theoretical possibility. 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
Leó Szilárd and
Eugene Wigner drafted the
Einstein–Szilárd letter, which warned of the potential development of "extremely powerful bombs of a new type". It urged the United States to take steps 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
Franklin D. Roosevelt. Roosevelt called on
Lyman Briggs of the
National Bureau of Standards to head the
Advisory Committee on Uranium to investigate the issues raised by the letter. Briggs held a meeting on 21 October 1939, which was attended by Szilárd, Wigner and
Edward Teller. 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."
The Advisory Committee on Uranium became the
National Defense Research Committee (NDRC) Committee on Uranium when that organization was formed on 27 June 1940.
 Briggs proposed spending $167,000 on research into uranium, particularly the
uranium-235 isotope, and the recently discovered
 On 28 June 1941, Roosevelt signed Executive Order 8807, which created the
Office of Scientific Research and Development (OSRD),
Vannevar Bush as its director. The office was empowered to engage in large engineering projects in addition to research.
 The NDRC Committee on Uranium became the S-1 Section of the OSRD; the word "uranium" was dropped for security reasons.
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), which was small enough to be carried by a bomber of the day.
 Their March 1940
Frisch–Peierls memorandum initiated the British atomic bomb project and its
 which unanimously recommended pursuing the development of an atomic bomb.
 In July 1940, Britain had offered to give the United States access to its scientific research, and the
John Cockcroft briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as far 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 United States in late August 1941 and discovered that data provided by the Maud Committee had not reached key American physicists. Oliphant then 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. To control the program, 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,
George C. Marshall. Roosevelt chose the Army to run the project rather than the Navy, because the Army had more experience with management of large-scale construction projects. He also agreed to coordinate the effort with that of the British, and on 11 October he sent a message to Prime Minister
Winston Churchill, suggesting that they correspond on atomic matters.