The United States Army Air Corps became the United States Air Force on what date?

The story behind the answer

September 18, 1947 is correct because the National Security Act of 1947 created the United States Air Force as an independent branch on that date, when the act took effect and W. Stuart Symington was sworn in as the first Secretary of the Air Force. Before that, American air power was part of the U.S. Army—the Air Corps (pre-1941) and then the U.S. Army Air Forces during World War II.

Context: The wartime success of strategic air power and the dawn of the Cold War drove this reorganization. The same act also formed the Department of Defense, the CIA, and the National Security Council. Memory tip: “September” starts with “Sep,” like “separate”—the Air Force separated from the Army on 9/18/1947.