Rare, Unseen: Building the Pentagon

The Pentagon -- and, by extension, the U.S. military -- has become such a prominent and obvious symbol of American might over the years that it's easy to forget that the world's largest office building and home of the Department of Defense is just that: a building. Here, on both the 10th anniversary of the 9/11 terror attack in Washington that left 125 Pentagon employees dead and, eerily, the 70th anniversary of the Pentagon's September 11, 1941, groundbreaking, LIFE.com presents rare and unpublished photographs of the iconic, colossal edifice under construction.