Szilard drafted a confidential letter to the President, Franklin D. Roosevelt, warning of a German nuclear weapon project, explaining the possibility of nuclear weapons, and encouraging the development of a program that could result in their creation.
With the help of Eugene Wigner and Edward Teller, he approached his old friend and collaborator Albert Einstein in August 1939, and convinced him to sign the letter, lending his prestige to the proposal.
The Einstein–Szilard letter resulted in the establishment of research into nuclear fission by the U.S. government.
An Advisory Committee on Uranium was formed under Lyman J. Briggs, a scientist and the director of the National Bureau of Standards.
Its first meeting on 21 October 1939 was attended by Szilard, Teller and Wigner.
The scientists persuaded the Army and Navy to provide $6,000 for Szilard to purchase supplies for experiments—in particular, more graphite.

In April 1941, the National Defense Research Committee (NDRC) created a special project headed by Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program.
Compton's report, submitted in May 1941, foresaw the prospects of developing radiological weapons, nuclear propulsion for ships, and nuclear weapons using uranium-235 or the recently discovered plutonium.
In October he wrote another report on the practicality of an atomic bomb.
For this report, he worked with Fermi on calculations of the critical mass of uranium-235.
He also discussed the prospects for uranium enrichment with Harold Urey.

Niels Bohr and John Wheeler had theorized that heavy isotopes with odd atomic mass numbers were fissile.
If so, then plutonium-239 was likely to be.
In May 1941, Emilio Segrè and Glenn Seaborg at the University of California produced 28 μg of plutonium in the 60-inch (150 cm) cyclotron there, and found that it had 1.7 times the thermal neutron capture cross section of uranium-235.
At the time only such minute quantities of plutonium-239 had been produced, in cyclotrons, and it was not possible to produce a sufficiently large quantity that way.
Compton discussed with Wigner how plutonium might be produced in a nuclear reactor, and with Robert Serber about how that plutonium might be separated from uranium.
His report, submitted in November, stated that a bomb was feasible.

The final draft of Compton's November 1941 report made no mention of plutonium, but after discussing the latest research with Ernest Lawrence, Compton became convinced that a plutonium bomb was also feasible.
In December, Compton was placed in charge of the plutonium project.
Its objectives were to produce reactors to convert uranium to plutonium, to find ways to chemically separate the plutonium from the uranium, and to design and build an atomic bomb.
It fell to Compton to decide which of the different types of reactor designs the scientists should pursue, even though a successful reactor had not yet been built.
He proposed a schedule to achieve a controlled nuclear chain reaction by January 1943, and to have an atomic bomb by January 1945.
Szilard drafted a confidential letter to the President, Franklin D. Roosevelt, warning of a German nuclear weapon project, explaining the possibility of nuclear weapons, and encouraging the development of a program that could result in their creation. With the help of Eugene Wigner and Edward Teller, he approached his old friend and collaborator Albert Einstein in August 1939, and convinced him to sign the letter, lending his prestige to the proposal. The Einstein–Szilard letter resulted in the establishment of research into nuclear fission by the U.S. government. An Advisory Committee on Uranium was formed under Lyman J. Briggs, a scientist and the director of the National Bureau of Standards. Its first meeting on 21 October 1939 was attended by Szilard, Teller and Wigner. The scientists persuaded the Army and Navy to provide $6,000 for Szilard to purchase supplies for experiments—in particular, more graphite.
In April 1941, the National Defense Research Committee (NDRC) created a special project headed by Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program. Compton's report, submitted in May 1941, foresaw the prospects of developing radiological weapons, nuclear propulsion for ships, and nuclear weapons using uranium-235 or the recently discovered plutonium. In October he wrote another report on the practicality of an atomic bomb. For this report, he worked with Fermi on calculations of the critical mass of uranium-235. He also discussed the prospects for uranium enrichment with Harold Urey.
Niels Bohr and John Wheeler had theorized that heavy isotopes with odd atomic mass numbers were fissile. If so, then plutonium-239 was likely to be. In May 1941, Emilio Segrè and Glenn Seaborg at the University of California produced 28 μg of plutonium in the 60-inch (150 cm) cyclotron there, and found that it had 1.7 times the thermal neutron capture cross section of uranium-235. At the time only such minute quantities of plutonium-239 had been produced, in cyclotrons, and it was not possible to produce a sufficiently large quantity that way. Compton discussed with Wigner how plutonium might be produced in a nuclear reactor, and with Robert Serber about how that plutonium might be separated from uranium. His report, submitted in November, stated that a bomb was feasible.
The final draft of Compton's November 1941 report made no mention of plutonium, but after discussing the latest research with Ernest Lawrence, Compton became convinced that a plutonium bomb was also feasible. In December, Compton was placed in charge of the plutonium project. Its objectives were to produce reactors to convert uranium to plutonium, to find ways to chemically separate the plutonium from the uranium, and to design and build an atomic bomb. It fell to Compton to decide which of the different types of reactor designs the scientists should pursue, even though a successful reactor had not yet been built. He proposed a schedule to achieve a controlled nuclear chain reaction by January 1943, and to have an atomic bomb by January 1945.
Chicago Pile-1