On September 11, 1941, ground was broken for the War Department's new headquarters in Arlington County, Virginia, across the Potomac from Washington. It was finished and occupied in January 1943, about sixteen months later, at a cost of roughly 83 million dollars. It was the largest office building in the world by floor area.
The building is five-sided because the site was. The plot available, bounded by roads, a river and the Arlington National Cemetery, was roughly a pentagon, and the shape followed it; a five-sided plan also shortened the walking distance between any two points, which mattered in a building designed for tens of thousands of people working on paper. It has five floors above ground, two basement levels, about 6.6 million square feet of floor space and around 28 kilometres of corridors. It fits into a footprint smaller than most of the Washington Mall.
Somervell drove it. The War Department was then scattered across seventeen buildings in Washington, plus temporary structures on the Mall, and he wanted it under one roof before it grew further. The architect was George Bergstrom. Construction ran around the clock. The building was designed before steel was rationed, which is why it is a reinforced-concrete frame with a limestone facade rather than a steel one. Almost 700,000 tons of sand and gravel were dredged from the Potomac.
The United States was not at war when the first shovel went in. Two months later, after Pearl Harbor on December 7, 1941, the building's size stopped looking wasteful and the department it served became the largest organization in the country by employment.
The Pentagon has been in continuous use since 1943. It was struck by a hijacked airliner on September 11, 2001, at the same time as the World Trade Center, in the section that had just been renovated, and 125 people in the building died along with everyone on the aircraft. It was repaired within a year.