From Washington Post:
ON A WARM AND RAINY THURSDAY EVENING IN JULY 1941, inside a War Department office in Washington, a small group of Army officers hastily assembled for a meeting and listened in disbelief to the secret plan outlined by their commander.
What's the secret plan? The largest office building in the world: The Pentagon.
Here's the story: Link | Photo Gallery (How The Pentagon Got Its Shape) - Thanks Mikolka!