May 23, 1942, marked the decision to proceed with all methods for isotope separation and plutonium production that had been developed to this point.
In June, 1942, Oppenheimer was appointed head of the bomb development group, and investigated the theoretical aspects of the design. Group members include Hans Bethe, Robert Serber, Emil Konopinksi, John Va Vleck, Felix Bloch, Richard Tolman, and Edward Teller. Tolmer suggests fission by implosion.
On September 17th, 1942, Leslie Groves was appointed military head of the project, at which point went about obtaining large amounts of high-grade uranium, created the Oak Ridge site for development of fissionable uranium, and gave the Manhattan Project the top secrecy rating. November, 1942, marked the acquisition and development of the Los Alamos site for development of the bomb platform.
In January, 1943, a large tract of land was acquired in Washington for the construction of the reactors that were to produce plutonium. February 1943 marked the construction of a Y-12 electromagnetic Uranium separation plant begins at Oak Ridge. The next month, construction of Oak Ridge's X-10 reactor site chemical separation plant began.