The Atomic Bomb Timeline

1931 — Harold C. Urey discovers deuterium (heavy hydrogen) which is present in all natural
hydrogen compounds including water. He later contributes to Uranium 235 isotope separation,
which is to be a key ingredient of the atomic bomb.
1932 — James Chadwick proves the existence of neutrons. Not being repelled by similarly
charged particles, the neutron made an ideal “bullet” for bombarding other nuclei to create a
nuclear fission reaction.
January 1933 — Adolph Hitler becomes chancellor of Germany. Due to anti-Semitism and Nazi
repression, many prominent scientists flee central Europe, including Albert Einstein.
September 1933 — Hungarian physicist Leo Szilard realizes that “if we could find an element
which is split by neutrons, and which would emit two neutrons when it absorbs one, such an
element would sustain a nuclear reaction.”
1934 — Enrico Fermi of Italy irradiates uranium with neutrons and unknowingly achieves the
world’s first nuclear fission.
June/July 1934 — Leo Szilard files a patent application for the atomic bomb. The patent
described the concept of using neutron induced chain reactions to create explosions.
December 1938 — Otto Hahn, Fritz Strassman and Lise Meitner produce uraninum nuclear fission
at Germany’s Kaiser Wilhelm Institute.
January 1939 — Otto Hahn and Fritz Strassmann in Berlin announce the discovery of uranium
fission. The observations are given their correct interpretation in a February 11 paper by Lise
Meitner and Otto Frisch.
February 1939 — Robert Oppenheimer speculates in a letter that a chain reaction in a 10-
centimeter cube of uranium deuteride "might very well blow itself to hell."
March 1939 — Fermi describes the recent fission experiments and their implication that uranium
could be a potent energy source or explosive to a group of U.S. military officials in Washington.
April 1939 — Paul Harteck and Wilhelm Groth in Hamburg inform German Army Weapons about
uranium fission, saying it may make possible powerful explosives.
September 1, 1939 — The German Army invades Poland; WWII begins.
September 16, 1939 — German Army Weapons recruits scientists for a wartime uranium project.
The meeting organizers are Kurt Diebner and his assistant, Erich Bagge, both Nazi party
members. Later that month Diebner convenes a second Army Weapons conference on uranium fission,
including Heisenberg, Weizsäcker, Harteck and Hahn. Heisenberg discusses a possible uranium
reactor, noting that with sufficiently enriched uranium it will explode. The fission program will be headquartered in the new Kaiser Wilhelm Institute of Physics (KWI) in Berlin-Dahlem. Fission research is made a secret of the state, and publication of scientific results is suppressed. Germany has a military nuclear-fission project.
December 1939 — Heisenberg submits the first part of a report to German Army Weapons,
concluding that the surest way to a reactor for energy production is enrichment of U235 in
uranium, also the only method of producing explosives "several orders of magnitude more
powerful than the strongest explosives yet known."
1940 — The University of California begins building a giant cyclotron under the direction of Ernest
O. Lawrence.
May 1940 — German troops in Norway seize control of the world's only heavy water production
facility and step up production to supply the German fission program.
June 1940 — Harteck reports on a uranium oxide-dry ice reactor. His 185 kilograms of uranium is
inadequate for neutron multiplication, but Heisenberg's group in Dahlem refuses to share more of
the available uranium; Heisenberg's Leipzig project has first claim on available heavy water.
July 1940 — Weizsäcker in a secret report to Army Weapons proposes that reactors can be used
to create neptunium for the construction of atomic bombs.
October 1940 — A new fission-research laboratory is completed at the KWI; Heisenberg
commutes between reactor experiments in Berlin and Leipzig.
March 1941 — Heisenberg and the Berlin-Dahlem team report on their first reactor experiments
with layers of uranium oxide and paraffin in a cylindrical tank. The results are negative, and
Heisenberg concludes that heavy water must be used.
June 22, 1941 — Germany invades the Soviet Union.
September 1941 — Heisenberg meets with Niels Bohr in Nazi-occupied Copenhagen and brings
up nuclear fission research. Heisenberg later describes the meeting as an attempt to seek
advice while Bohr sees it as a hostile approach. In Copenhagen Heisenberg and Weizsäcker
lecture at a Nazi propaganda institute.
December 1941 — The head of Army Weapons Research orders a review of the uranium project.
Declaring the army can no longer support projects that will not yield results in the foreseeable
future, he considers cancellation of support for fission research.
December 8, 1941 – The United States declares war on Japan one day after the
bombing of Pearl Harbor.
December 1941 — President Roosevelt allocates $2 billion to the Manhattan Engineering District
to build the atomic bomb.
January 1942 — Heisenberg and collaborators report to Army Weapons on reactor experiment B-
III. Still without heavy water, the reactor uses paraffin and uranium powder and produces no
neutron multiplication.
September, 1942 — Colonel Leslie Groves is promoted to Brigadier-General and put in charge of
the Manhattan Project. He recruits J. Robert Oppenheimer as Scientific Director.
February 1942 — The Reich Research Council holds a conference in Berlin to promote uranium
research to an audience of Nazi leaders. Heisenberg explains how a successful reactor might
be used in submarines, that U235 can be used to make a bomb and that a reactor could
generate plutonium. No critical-mass figure is given, but a mass of tons is implied.
March 1942 — Albert Speer places the German economy on a war footing. Projects that do not
promise short-term results are eliminated or down-graded in priority.
April 1942 — The Leipzig L-IV reactor demonstrates neutron generation of 13 percent. Heisenberg
predicts this design could be critical with 5 metric tons of heavy water and 10 tons of solid
uranium metal. The reactor is destroyed by fire in June. Heisenberg leaves for Berlin, and
reactor research ceases in Leipzig.
June 1942 — A secret meeting is held in Dahlem including War Minister Speer and leading
nuclear scientists. Heisenberg describes atomic bombs as possible but not in the near future.
Speer approves all the scientists' requests, including a bomb-resistant bunker for a large reactor,
but the project receives the lowest priority that allows it to proceed.
July 1942 — Heisenberg becomes acting head of the KWI in Dahlem and begins planning a
series of large reactor experiments involving as much of Germany's uranium and heavy water as
possible. The uranium metal plates of his design prove difficult to manufacture as well as
ineffective; while awaiting delivery he turns to other research.
December 1942 — At the University of Chicago, in a squash court under Stagg Field, Enrico Fermi
and his team produce the world’s first controlled and self-sustained nuclear fission reaction.
November 1943 — The U.S. Military begin remodeling the B-52 bomber for delivery of the A-bomb.
May 1945 — Germany surrenders. The war in Europe is over.
July 1945 — Trinity Test at Alamogordo, New Mexico. The A-bomb explodes with an 18,000 ton
TNT equivalence.
August 6 and 9, 1945 — “Little Boy” explodes over Hiroshima, killing over 100,000
people. Three days later, “Fat Man” explodes over Nagasaki, killing over 75,000.
Japan surrenders soon after.

Compiled from Doug Prouty’s “The Race to Build the Atomic Bomb: A Resource for Teachers and Students”, Contra Costa County Office of Education, at and the archives of American Scientist, the Magazine of Sigma Xi (The Scientific Research Society) at

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