The Manhattan Project, An Interactive History Home The Manhattan Project, An Interactive History Home Department of Energy Home Office of History and Heritage Resources Home DOEHome
J.R. Oppenheimer and General Groves
Events People Places Processes Science Resources

Time Periods

Atomic Discoveries

Government Support


The Uranium
Path to
the Bomb

The Plutonium
Path to
the Bomb

Bringing It All Together

Dawn of the
Atomic Era

Postscript --
The Nuclear Age

President Roosevelt signs declaration of war with Japan, December 8, 1941.FINAL APPROVAL TO BUILD THE BOMB
(Washington, D.C., December 1942)
Events > Difficult Choices, 1942

Anxious as he was to get moving, Leslie Groves decided to make one final quality control check.  On November 18, 1942, Groves appointed Warren K. Lewis of the Massachusetts Institute of Technology to head a final review committee, comprised of himself and three DuPont representatives.  During the final two weeks of November, the committee traveled from New York to Chicago to Berkeley and back again through Chicago.  It endorsed the work on gaseous diffusion at Columbia, though it made some organizational recommendations; in fact, the Lewis committee advocated elevating gaseous diffusion to first priority and expressed reservations about the electromagnetic program despite an impassioned presentation by Ernest Lawrence in Berkeley.  Upon returning to Chicago, Crawford H. Greenewalt, a member of the Lewis committee, was present at Stagg Field when CP-1 (Chicago Pile #1) first went critical.  (For more on CP-1, skip ahead to "Early Pile Design, 1942.")  Significant as this moment was in the history of physics, it came after the Lewis committee endorsed moving piles to the pilot stage and one day after Groves instructed DuPont to move into pile design and construction.  

S-1 Committee, Bohemian Grove, September 13, 1942.The S-1 Executive Committee (left) met to consider the Lewis report on December 9, 1942.  Most of the morning session was spent evaluating the controversial recommendation that only a small electromagnetic plant be built.  Lewis and his colleagues based their recommendation on the belief that Lawrence could not produce enough uranium-235 to be of military significance.  But since Lawrence's calutrons could provide enriched samples quickly, the committee supported the construction of a small electromagnetic plant.  James Conant disagreed with the Lewis committee's assessment, believing that uranium had more weapon potential than plutonium.  And since he knew that gaseous diffusion could not provide any enriched uranium until the gaseous diffusion plant was in full operation, he supported the one method that might, if all went well, produce enough uranium to build a bomb in 1944.  During the afternoon, the S-1 Executive Committee went over a draft Groves had prepared for Vannevar Bush to send to President Franklin D. Roosevelt.  The draft supported the Lewis committee's report except that it recommended skipping the pilot plant stage for the pile.  After Conant and the Lewis committee met on December 10 and reached a compromise on an intermediate-scale electromagnetic plant, Groves's draft was amended and forwarded to Bush.  

Vannevar Bush and Arthur Compton, 1940On December 28, 1942, President Roosevelt approved what ultimately became a government investment in excess of $2 billion, $0.5 billion of which was itemized in Bush's report submitted on December 16.  The Manhattan Project was authorized to build full-scale gaseous diffusion and plutonium plants and the compromise electromagnetic plant, as well as heavy water production facilities.  In his report, Bush reaffirmed his belief that bombs possibly could be produced during the first half of 1945 but cautioned that an earlier delivery was unlikely.  No schedule could guarantee that the United States would overtake Germany in the race for the bomb, but by the beginning of 1943 the Manhattan Project had the complete support of President Roosevelt and the military leadership, the services of some of the nation's most distinguished scientists, and a sense of urgency driven by fear.  Much had been achieved in the year between Pearl Harbor and the end of 1942.  

Franklin Roosevelt's note to Vannevar Bush giving Bush the tentative go-ahead to build the atomic bomb, January 19, 1942.No single decision created the American atomic bomb project.  Roosevelt's December 28 decision was almost inevitable in light of numerous earlier ones that, in incremental fashion, committed the United States to the pursuit of atomic weapons.  In fact, the essential pieces were in place when Roosevelt approved Bush's November 9, 1941 report on January 19, 1942 (left).  At that time, there was a science organization at the highest level of the federal government and a Top Policy Group with direct access to the President.  Funds were authorized, and the participation of the Corps of Engineers had been approved in principle.  In addition, the country was at war and its scientific leadership -- as well as its President -- had the belief, born of the MAUD Report, that the project could result in a significant contribution to the war effort.  Roosevelt's approval of $500 million in late December 1942 was a step that followed directly from the commitments made in January of that year and stemmed logically from the President's earliest tentative decisions in late 1939.  

To view the next "event" of the Manhattan Project, proceed to "1942-1944: The Uranium Path to the Bomb."


Sources and notes for this page.

The text for this page was adapted from, and portions were taken directly from the Office of History and Heritage Resources publication: F. G. Gosling, The Manhattan Project: Making the Atomic Bomb (DOE/MA-0001; Washington: History Division, Department of Energy, January 1999), 16-17.  For more on the Lewis Committee Report, see the History Office publication: Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946: Volume I, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972), 113.  The photograph of Franklin Roosevelt is courtesy the National Archives.  Click here for more information on the photograph of the S-1 (Uranium) Committee.  The photograph of Vannevar Bush and Arthur Compton is courtesy the Lawrence Berkeley National Laboratory.  The note from Roosevelt to Bush is available on the National Archives microfilm collection M1392, Bush-Conant File Relating to the Development of the Atomic Bomb, 1940-1945 (Washington: National Archives and Records Administration, 1990), reel #1/14.

Home | History Office | OpenNet | DOE | Privacy and Security Notices
About this Site | How to Navigate this Site | Note on Sources | Site Map | Contact Us