In June 1940, despite the U.S. not being at war, the fall of France and the Low Countries triggered a far-reaching American military mobilization effort. The first-ever peacetime draft increased the standing army of 174,000 soldiers eightfold to 1.4 million by June 1941. This kicked off a crash program of military construction, with Secretary of War Henry L. Stimson tapping Lt. Col. Brehon B. Somervell to lead it. He had led the New York City Works Progress Administration, overseeing 200,000 workers as well as the construction of LaGuardia Airport. Taking command of the Army’s construction division in December 1940, he finished building 50 major camps, 9 hospitals and 45 munitions plants, plus ports, chemical warfare plants and storage depots within months.

With 24,000 War Dept. military and civilian staffers scattered across Washington, D.C., in two dozen buildings and hundreds more hired each week, military leadership fixed its sights on a site across the Potomac River in Arlington, Va., for a new headquarters. On Thursday, July 17, 1941, Somervell summoned his design team leaders—Lt. Col. Hugh “Pat” Casey, an engineer, and George Edwin Bergstrom, president of the American Institute of Architects—tasking them to design four-story building to hold 40,000 workers by Monday, which they did.

The site was five-sided, which led the team to pick the building’s unconventional shape. They settled on a set of five concentric, five-sided rings, each with a 921-ft-long outer wall, enclosing a six-acre central court. It would be the world’s largest office building. “Pedestrian circulation within the building is by means of ramps leading from floor to floor, and by radial corridors on each floor that branch out much as spokes of a wheel from the main corridors encircling the inner court,” wrote ENR in 1942. Reinforced concrete was chosen to reduce the amount of steel, badly needed for the war effort, by more than half.

Construction began Sept. 11, with earthmoving and the casting of the first of an eventual 41,492 reinforced concrete piles. By mid-January 1942 there were 6,000 workers placing concrete slabs, hanging limestone slabs on the facade and operating a forest of cranes and tower hoists. Barges of gravel heading upriver to private customers were met by boats carrying officers from the project, who requisitioned them on the spot. Teams of carpenters, working in assembly lines, built forms for the columns, beams, slabs and walls. The workforce peaked at 13,000 that summer. Army Chief of Staff George C. Marshall later reorganized the Army, promoting Somervell to commander of all Army supply forces worldwide, while also continuing to oversee Pentagon construction.

More than 30 miles of highways, 21 overpasses, and some of the first cloverleafs in the U.S. were built to allow workers and materials to reach the site. Most employees arrived on buses, which loaded and unloaded at a terminal in the basement. Two massive parking lots, covering 67 acres, had spaces for 8,000 cars. The world’s largest telephone switchboard was built, occupying 32,000 sq ft. There were 300 switchboard operators and 27,000 phones, with every call connected manually. Messengers on bicycles, roller skates and oversize tricycle carts traveled the corridors, ringing bells to avoid hitting pedestrians. 

The building had separate bathrooms and dining halls for blacks and whites, in adherence to Virginia’s racial laws. But the segregation ended within the first month after black workers protested vigorously.

To mollify budget hawks, Somervell had fudged the Pentagon’s true size, pegging it as 4 million sq ft through design and construction. But it actually was 6.2 million sq ft, partially driven by the decision mid-construction to add a fifth floor. 

Given the rapid recruitment of workers and construction pace, safety practices were sometimes lax. Eight workers died during the project.

The final project cost was $75 million, equivalent to $1.4 billion today. Somervell had promised military brass and members of Congress completion in 14 months and that workers could begin moving in within six months of groundbreaking. Those goals slipped only slightly: staffers were at their desks within seven months, 10,000 there by July 1942. Final completion was in February 1943, 17 months afterwork started.