Pentagon

On September 11, 1941, construction of the Pentagon began in Arlington County, Virginia. It was designed by architect George Bergstrom and built by contractor John McShain. The construction was overseen by Leslie Groves who was later put in charge of the Manhattan Project. From the article:

"The Pentagon is the Virginia headquarters of the U.S. Department of Defense, located in a massive five-sided concrete and steel building that’s a potent symbol of America’s military strength. With more than 6 million square feet of floor space, the Pentagon ranks among the largest office buildings in the world. During the terrorist attacks of September 11, 2001—60 years to the day after construction began on the Pentagon—a hijacked plane struck the building, killing 189 people and damaging roughly one-third of the building."