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Serial number stencil on the forward port section of the aircraft. It reads

U.S. Army - Model XB-47
Air Forces Serial No. 46-66
Crew Weight 500 lbs.

The U.S. Air Force was part of the U.S. Army until September 18, 1947, when the National Security Act of 1947 went into effect and established the USAF an independent branch of the U.S. armed forces.