Chronology for the Development of
the Atomic Bomb and Its Use Against Japan

1895: X-rays discovered by Wilhelm Conrad Roentgen (Germany).

1896: Radioactivity discovered by Henri Becquerel and named by Marie Curie (France.)

1905: Theory of relativity presented by Albert Einstein (Germany).

1911: Ernest Rutherford establishes the nuclear theory of the atom (England).

1914: H. G. Wells publishes “The World Set Free” in which he prophesizes the creation of an atomic weapon (England).

1932: Ernest Orlando Lawrence and M. S. Livingston invent the cyclotron to accelerate sub-atomic particles (United States).

1933: Adolf Hitler becomes Chancellor of Germany. Jews are denied university posts and many physicists and other scientists flee to the United States and England. In October, Leo Szilard hits upon the idea of a chain reaction (England).

1934: Enrico Fermi produces fission (Italy).

1935: Edward Teller seeks refuge in the United States. Leo Szilard takes out secret patent on chain reaction (England).

1938: Otto Hahn and Fritz Strassmann achieve nuclear fission with uranium (Germany). Otto Hahn sends paper to Lise Meitner to see if she concurs that he has split the uranium atom. Meitner and Otto Frisch confirm nuclear fission (Sweden).


January 1: Enrico Fermi, an Italian scientist whose wife is Jewish, seeks refuge in the United States after receiving the Nobel Prize.

January 17: Otto Frisch submits a paper on his experiments with nuclear fission of uranium (Denmark).

January 26: The American Physics Society sponsors a meeting to discuss the development with Niels Bohr, Enrico Fermi, Otto Stern, H. C. Urey, Isidor Isaac Rabi, and Edward Teller.

January 30: Robert Oppenheimer predicts the possibility of a nuclear bomb.

March 17: The first meeting on using nuclear energy for military purposes occurs when Enrico Fermi talks with the United States Navy about the development of a nuclear bomb.

August 2: Leo Szilard, Edward Teller, and E. Wigner persuade Albert Einstein, now living in the United States, to warn the president about the possibility of nuclear weapons. Albert Einstein writes President Franklin D. Roosevelt to alert him about the importance of doing research on nuclear weapons so that the United States would beat Germany in the race for the bomb.

August 19: FDR informs Einstein he has set up a committee to study uranium.

September 1: Germany invades Poland and World War II begins in Europe.

September 16: A secret conference is held in Berlin to discuss nuclear fission.

September 26: The German nuclear bomb project is organized with Werner Heisenberg, Paul Harteck, Kurt Diepner, and Erich Bagge.

October 11-12: Alexander Sachs speaks with FDR about Einstein’s letter and the necessity of getting an atomic bomb before the Germans. FDR understands the urgency, though real funding will not be provided for many more months.

October 21: First meeting of the Uranium Committee (United States).

December 6: Werner Heisenberg makes first report to the German Department of War on atomic weapons.


February 1: Otto Frisch and E. Peierls present idea of achieving critical mass by using highly enriched uranium (England).

February 2: E. Fermi and L. Szilard given $6000 by U.S. government to purchase graphite. February 29: A. Nier separates uranium 235.

April 9: Germany invades and occupies Denmark.

May 3: German troops occupy Norway, seizing the world’s only production plant for heavy water.

May 10, 1940: FDR appeals for cooperation to the American Scientist Federation. E. Teller decides on the basis of this speech to dedicate himself to weapons research.

June: H. Halban and L. Kowarski escape to England carrying 180 pounds of heavy water and all the data of Jolie Curie. Shortly after, Paris falls to the Germans and Germany invades the Soviet Union. Vannevar Bush is named head of the National Defense Research Committee.

Spring-Summer: Separation of isotopes studied in the United States and with the MAUD committee in England.


February 23: Glenn Seaborg discovers a new element and names it plutonium (United States).

March: R. Peierls calculates critical mass of uranium 235 (England).

April: Japanese army approves beginning of atomic bomb research.

May: Klaus Fuchs, a German émigré physicist, begins research on atomic bomb in England. Fuchs will communicate atomic discoveries to the Soviets throughout the war.

June 22: Germany invades the Soviet Union. Shortly before, the Soviets had established their own committee to investigate the possibility of an atomic weapon.

June 23: MAUD committee presents convincing evidence that uranium 235 can be made into a bomb.

July 7: The MAUD committee findings and its scientists are transferred to the United States, the only country thought to have the resources to develop the atomic bomb.

July 11: Ernest Lawrence suggests possibility of fission of plutonium, thus creating a plutonium bomb.

November 6: The Compton committee, headed by Arthur Compton, predicts that a uranium bomb was possible within 3-4 years at the cost of $50-$100 million dollars, an estimate that turns out to be too conservative.

December 6: On the day before Pearl Harbor, FDR authorizes the establishment of the Manhattan Engineering Project under the Office of Scientific Research and Development.

December 7: Japan attacks the United States at Pearl Harbor and Germany follows by a declaration of war against the United States. World War II begins in earnest for America.

December 9: Arthur Compton and Vannevar Bush begin plutonium bomb project.

December 18: The S-1 Committee is created by Vannevar Bush with a 6 month budget of $651,000 to develop a nuclear bomb. Ernest Lawrence is on the S-1 Committee and shortly afterwards builds the Calutron (named after his institution, University of California) to separate U235 and make it useable for a bomb.


January 24: Atomic research concentrated at the University of Chicago in the Metallurgical Laboratory with Arthur Compton, Ernest Lawrence, Leo Szilard, and others.

June 11: Robert Oppenheimer sent to the Metallurgical Laboratory at University of Chicago.

June 13: FDR told that a plutonium bomb is feasible by Vannevar Bush and James Conant.

June 17: Plutonium begins to be produced at St. Louis University and Washington University.

June 25: The first S-1 Committee meeting is held with key scientists and military officials.

June 27: Arthur Compton encourages scientists to work on nuclear program at a Conference at Chicago’s Metallurgical Laboratory.

July 27: First plutonium arrives at Chicago Metallurgical Laboratory.

August: The Army Corps of engineers creates a new District organization with the intentionally misleading name “Manhattan Engineer District”.

September: Igor Kurchatov is made head of the Soviet bomb project.

September 17: Colonel Leslie Groves, who had completed construction of the Pentagon, is informed that he has been made a Brigadier General and put in charge of the Manhattan Engineer District.

September 26: At Groves’ insistence, the Manhattan Project is given the highest emergency procurement priority in existence (AAA) allowing Groves to acquire materials for the Manhattan Project over any other project underway.

October 5: Groves visits the Metallurgical Laboratory at the University of Chicago and meets key scientists for the first time, including Leo Szilard and Robert Oppenheimer.

October 15: Groves asks Oppenheimer to be head of Project Y, a new central laboratory for physics research on weapons. This will be moved to Los Alamos and become an important part of the Manhattan Project.

November 5: Construction of a uranium isotope separation plant begins at Oak Ridge, Tennessee under General Leslie Groves.

December 2: Enrico Fermi at the University of Chicago produces the first sustained and controlled nuclear reaction. Soon after, Bush gets FDR to approve the expenditure of $400 million to develop the bomb.


January: Planning begins for construction of reactors at Hanford, Washington to make plutonium for bombs under the direction of General Leslie Groves.

March 15: Robert Oppenheimer moves the Manhattan Project to a secret laboratory at Los Alamos, New Mexico.

April: The fundamental organization of Los Alamos is worked out with Oppenheimer and other scientists insisting upon open communication despite Groves’ desire for compartmentalization and secrecy. Seth Neddermeyer begins research on implosion. Concurrently, explosion techniques are worked on. The University of California is contracted with to manage Los Alamos, as it still does.

July 4: Neddermeyer conducts the first implosion experiments.

August: 20,000 people are working on construction at Oak Ridge, Tennessee. 5,000 are at work constructing the Hanford, Washington facility.

September 20: Johann Vonn Neumann visits Los Alamos and points out more efficient way to achieve implosion with a smaller critical mass. Research on implosion speeds up and Oppenheimer recruits George Kistiakowsky to lead the explosives research.

October: Project Alberta, the atomic bomb delivery program, begins under Norman Ramsey and plans to modify aircraft for the delivery of nuclear bombs begins.

November: Klaus Fuchs, the Soviet spy, arrives at Los Alamos with other British scientists.


January 11: As Seth Neddermeyer’s shortcomings become apparent, he is replaced by George Kistiakowsky and Edward Teller to oversee implosion research and development.

April 5: The first plutonium from Oak Ridge arrives at Los Alamos.

May: Los Alamos staff exceeds 1200 employees. Teller is removed from his position with implosion research because of personality conflicts and his increasing obsession with the Super (hydrogen bomb).

June: Oppenheimer puts himself in charge of implosion research.

July: Experiments with using a lens to trigger explosion begin and the design for the gun gadget neutron initiator is completed. August: The Air Force begins modifying 17 B-29s to deliver atomic weapons.

September: Air Force Lt. Col. Paul Tibbets begins organizing the 509th Composite Group which will deliver atomic bombs in combat, though most of the men do not know their secret mission.

October 12: The first B-29s arrive in the Mariana Islands to begin bombing Japan.

October 27: Oppenheimer approves plans for a test of the bomb in the Jornada del Mueurto (Journey of Death) valley of the Alamagordo Bombing Range.

November 24: The first B-29 raid on Japan is conducted.

December: First successful explosive lens test conducted at Los Alamos, establishing the feasibility of an implosion bomb.


January 18: The experiment of “tickling the Dragon’s tail” is conducted by Otto Frisch with U-235 going critical.

January 20: Curtis LeMay takes command of the Twentieth Air Force in the Marianas.

February: The gun device for the uranium bomb is completed. Admiral Nimitz, Commander in Chief, Pacific Ocean Areas, is notified about the nature of the atomic bomb project.

February 13: Dresden, Germany is bombed in an incendiary raid, killing 50,000 people.

February 19: Marines land on Iwo Jima; more than 6,000 of them will be killed in the next two months.

March 1: The battle for Okinawa begins. The Cowpuncher Committee is organized to “ride herd” on the development of an implosion bomb.

March 9-10: The Twentieth Air Force launches low altitude fire bomb raid on Tokyo, killing approximately 83,000 people. This is the most destructive air raid in history.

April 12: President Roosevelt dies of a brain hemorrhage and Harry Truman becomes president. The Riken Scientific Institute in Tokyo is destroyed and with it the Japanese atomic program.

April 13: Truman is informed by Secretary of War Harry Stimson of the existence of an atomic bomb project.

April 17: The ALSOS unit, sent by Groves to capture men and materials involved in the German atomic effort, finds German uranium ore and sends it to Los Alamos.

April 23: ALSOS seizes German reactor at Heigerloch.

April 24: ALSOS arrests German atomic scientists, including Werner Heisenberg. It becomes clear Germany was nowhere near developing an atomic bomb. Groves decides not to share this information with the scientists at Los Alamos, recognizing that many of them had devoted themselves to weapons research against Germany, not against Japan.

April 25: Truman receives detailed report on the Manhattan Project from Stimson and Groves.

April 27: The first meeting of the Target Committee is held to select targets for atomic bombing. Hiroshima and Nagasaki are among 17 possible cities on the list.

May 8, 1945: Germany surrenders and Victory in Europe Day is declared.

May 10-11: Target Committee meets again with Oppenheimer now on the committee.

May 25: 464 B-29s raid Tokyo again and 16 square miles of the city are destroyed. “Operation Olympiad” planned for the invasion of the Japanese home islands and scheduled for November 1, 1945.

May 28: Target Committee meets with Lt. Col. Tibbets in attendance. Tibbets estimates by January 1, 1946 all the major cities in Japan will have been destroyed by fire bombing. The atomic target list is now Kyoto, Hiroshima, and Niigata.

May 30: Secretary of War Stimson rules out Kyoto, the cultural capital of Japan, for atomic attack.

May 31: Critical mass tests with plutonium begin at Los Alamos.

June 1: The Interim Committee, led by Secretary of State Designate James Byrnes, recommends that the atomic bomb be dropped as soon as possible, without warning, and that an urban target be selected.

June 2: Klaus Fuchs gives confidential material to Soviets.

June 11: The Franck Committee report recommends notification before use of the atomic bomb. Seven scientists in Chicago recommend a demonstration before the bomb is used on a city.

June 18: George Marshall estimates 63,000 Americans will be killed or wounded in Operation Olympiad. (Other estimates were as high as 1,000,000 Allied casualties.)

June 21: Combined U.S. and British force defeats the Japanese on Okinawa with unprecedented Allied, civilian and Japanese losses.

July 11: Assembly of the Gadget, the first uranium bomb, begins. The Japanese begin trying to get Moscow to act as an intermediary for their surrender, but the Soviets mislead and betray them.

July 13: The Gadget is assembled.

July 14: The Gadget is hoisted to the top of a 100 foot test tower.

July 16, at 5:29:45 a.m.: the Gadget is detonated in the first atomic explosion in history. Truman receives notification of successful test while at the Potsdam Conference with Winston Churchill and Joseph Stalin. Churchill refers to the atomic bomb as “The Second Coming, in wrath.” Four hours after the test, the cruiser Indianapolis begins shipping Little Boy to the Tinian Islands for use against Japan.

July 24: Truman tells Stalin about the atomic bomb in vague terms. Stalin is already aware of the bomb because of his spies. Groves authorizes use of atomic bombs as soon as they are available and weather permits. The following targets are selected in order of priority: Hiroshima, Kokura, Niigata, and Nagasaki. No further orders are needed.

July 26: Truman issues the Potsdam Proclamation, demanding that the Japanese surrender unconditionally, with no guarantee that Emperor Hirohito would remain ruler.

July 28: The Japanese reject the Potsdam surrender demand.

July 31: The assembly of the uranium bomb, Little Boy, is completed.

August 1: A typhoon approaching Japan delays the atomic attack for several days.

August 4: Tibbets briefs 509th Composite Group about the impending attack, revealing that they will be dropping a new and powerful bomb, though they are not told that it is atomic.

August 5: Tibbets names his plane the “Enola Gay” after his mother. Little Boy is loaded on the plane.

August 6: Hiroshima is chosen as the first target. The bomb is armed after the planes lifts off from Tinian. At 8:16:02 Hiroshima time, Little Boy (uranium bomb) is released and explodes at an altitude of 1850 feet, 550 feet from the target, the Aioi Bridge. Two-thirds of the city is destroyed and 66,000 people are killed by the blast with 69,000 more injured.

August 9: Fat Man (plutonium bomb) is loaded onto “Bock’s Car,” the B-29 selected for the next atomic attack, and takes off to bomb Kokura. Because that target is obscured, Nagasaki is bombed instead at 11:02 Nagasaki time, exploding 1950 feet over the city. 40% of the city is destroyed and around 35,000 killed immediately. The Soviet Union declares war on Japan and invades Japanese controlled territory in Manchuria.

August 10: Emperor Hirohito breaks with tradition and orders surrender of Japan.

August 11: Groves plans for next atomic attack on August 17 or 18th, but Truman orders a halt to further atomic bombings and fire bombing is also stopped.

August 13: With official surrender still not accomplished, Truman orders fire-bombing resumed. Over 1000 B-29s attack throughout Japan, the largest raid of the war.

August 14: Emperor Hirohito issues Imperial Edict accepting surrender.

August 15: World War II ends, a conflict that had killed between 40 and 50 million people.

September 2: Japanese emissaries sign document of surrender on board the battleship Missouri in Tokyo Bay.



  • The discovery of nuclear fission and the development of the atomic bomb was an international effort.
  • Because some of the earliest discoveries occurred in Germany, there was concern that Germany would get the atom bomb before the Allies.
  • Though the project got off to a slow start in the United States and was inadequately funded for the first 18 months, during the last two years of the war, the American government invested enormous manpower and money into designing a bomb for military use.
  • Because the United States was the only country that could offer scholarly appointments and funds to the many scientists being persecuted or expelled under totalitarian regimes, all Allied research was conducted in the U.S.
  • Many people contributed to the success of the bomb, but two people, Robert Oppenheimer and Leslie Groves, were critical to the project because of their remarkable administrative and technical abilities.
  • Almost all of the time of the scientists and military people was taken up with making the bomb; very little time was given to the question of whether and how to use the bomb. For the scientists involved, it was only in the spring of 1945 that questions began to be raised.
  • Germany was never close to developing an atomic weapon, and Japan’s program never progressed past infancy. American scientists did not know about the German failure until the spring of 1945.
  • This chronology assembled by Kathleen Minnix and edited by James Frieden of