During the Manhattan Project, Washington, DC was home to the offices of General Leslie Groves, the leader of the project. Initially headquartered in Manhattan, Groves moved project headquarters to the nation's capital upon his appointment to lead the project on Sept. 17, 1942. Operating out of a small two-room office in the New War Building (now the location of the Department of State), Groves and a handful of staff directed the far-reaching top-secret project and the thousands of personnel it employed.
Groves, who had previously been tasked to oversee construction of the Pentagon just across the Potomac River, had chosen to remain in Washington, DC to be in proximity to the War Department and other agencies that were involved in the project, although his role as project leader required repeated travel to top-secret locations, including Hanford, Los Alamos, and Oak Ridge.
Today, Washington, DC is home to the National Archives and Records Administration, which houses still-classified documents related to the Manhattan Project.
Source: U.S. Department of the Interior, National Park Service