Construction begins on the Pentagon.

The Pentagon is the headquarters building of the United States Department of Defense, in Arlington County, Virginia, across the Potomac River from Washington, D.C. The building was constructed on an accelerated schedule during World War II. As a symbol of the U.S. military, the phrase The Pentagon is often used as a metonym for the Department of Defense and its leadership.

September 11 is the 254th day of the year in the Gregorian calendar; 111 days remain until the end of the year.