The Manhattan Project Timeline

Mushroom cloud over Hiroshima During World War II
Mushroom cloud over Hiroshima During World War II. United States Air Force

The Manhattan Project was a secret research project that was created to help America design and build an atomic bomb. This was created in reaction to Nazi scientists who had discovered how to split a uranium atom in 1939. In fact, President Franklin Roosevelt was not that concerned when Albert Einstein first wrote him about the possible consequences of splitting the atom. Einstein had previously discussed his concerns with Enrico Fermi who had escaped from Italy.

However, by 1941 Roosevelt had decided to create a group to research and develop the bomb. The project was given its name due to the fact that at least 10 of the sites used for the research were located in Manhattan. Following is a timeline of the key events related to the development of the atomic bomb and the Manhattan Project. 

Manhattan Project Timeline

1931Heavy Hydrogen or deuterium is discovered by Harold C. Urey.
1932The atom is split by John Crockcroft and E.T.S. Walton of Great Britain thereby proving Einstein's Theory of Relativity.
1933Hungarian physicist Leo Szilard realizes the possibility of the nuclear chain reaction.
1934The first nuclear fission is achieved by Enrico Fermi of Italy.
1939The Theory of Nuclear Fission is announced by Lise Meitner and Otto Frisch.
January 26, 1939At a conference at George Washington University, Niels Bohr announces the discovery of fission.
January 29,1939Robert Oppenheimer realizes the military possibilities of nuclear fission.
August 2, 1939Albert Einstein writes to President Franklin Roosevelt concerning the use of uranium as a new source of energy leading to the formation of the Committee on Uranium.
September 1, 1939World War II Begins.
February 23, 1941Plutonium is discovered by Glenn Seaborg.
October 9, 1941FDR gives the go-ahead for the development of an atomic weapon.
December 6, 1941FDR authorizes the Manhattan Engineering District for the purpose of creating an atomic bomb. This would later be called the 'Manhattan Project'.
September 23, 1942Colonel Leslie Groves is placed in charge of the Manhattan Project. J. Robert Oppenheimer becomes the Project's Scientific Director.
December 2, 1942First controlled nuclear fission reaction is produced by Enrico Fermi at the University of Chicago.
May 5, 1943Japan becomes the primary target for any future atomic bomb according to the Military Policy Committee of the Manhattan Project.
April 12, 1945Franklin Roosevelt dies. Harry Truman is named the 33rd President of the US.
April 27, 1945The Target Committee of the Manhattan Project select four cities as possible targets for the atomic bomb. They are: Kyoto, Hiroshima, Kokura, and Niigata.
May 8, 1945War ends in Europe.
May 25, 1945Leo Szilard attempts to warn President Truman in person concerning the dangers of atomic weapons.
July 1, 1945Leo Szilard begins a petition to get President Truman to call off using the atomic bomb in Japan.
July 13,1945American intelligence discovers the only obstacle to peace with Japan is 'unconditional surrender'.
July 16, 1945The world's first atomic detonation takes place in the 'Trinity Test' at Alamogordo, New Mexico.
July 21, 1945President Truman orders atomic bombs to be used.
July 26, 1945Potsdam Declaration is issued, calling for the 'unconditional surrender of Japan'.
July 28, 1945Potsdam Declaration is rejected by Japan.
August 6, 1945Little Boy, a uranium bomb, is detonated over Hiroshima, Japan. It kills between 90,000 and 100,000 people immediately. Harry Truman's Press Release
August 7, 1945U.S. decides to drop warning pamphlets on Japanese cities.
August 9, 1945The second atomic bomb to hit Japan, Fat Man, was scheduled to be dropped at Kokura. However, because of poor weather the target was moved to Nagasaki.
August 9, 1945President Truman addresses the nation.
August 10, 1945U.S. drops warning leaflets concerning another atomic bomb on Nagasaki, the day after the bomb was dropped. 
September 2, 1945Japan announces its formal surrender.
October, 1945Edward Teller approaches Robert Oppenheimer to aid in the building of a new hydrogen bomb. Oppenheimer refuses.