The Pentagon - Wikipedia

On September 11, 1941, construction of the Pentagon began. It was dedicateded 15 Jan 1943. From the article:

"The Pentagon was designed by American architect George Bergstrom (1876–1955), and built by general contractor John McShain of Philadelphia. Ground was broken for construction on September 11, 1941, and the building was dedicated on January 15, 1943. General Brehon Somervell provided the major motivating power behind the project;[5] Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army.

The Pentagon is the world's largest office building, with about 6,500,000 sq ft (600,000 m2), of which 3,700,000 sq ft (340,000 m2) are used as offices.[6][7] Approximately 23,000 military and civilian employees[7] and about 3,000 non-defense support personnel work in the Pentagon. It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 mi (28.2 km)[7] of corridors. The Pentagon includes a five-acre (20,000 m2) central plaza, which is shaped like a pentagon and informally known as "ground zero," a nickname originating during the Cold War on the presumption that it would be targeted by the Soviet Union at the outbreak of nuclear war.[8]

On September 11, 2001, exactly 60 years after the building's construction began, American Airlines Flight 77 was hijacked and flown into the western side of the building, killing 189 people (59 victims and the five perpetrators on board the airliner, as well as 125 victims in the building), according to the official report.[9] It was the first significant foreign attack on Washington's governmental facilities since the city was burned by the British during the War of 1812."