Question: When did the army air corp become the air force?
Disambiguated questions and answers: question: When did the army air Corp become the United States Army Air Forces?
When was the army air corp legally abolished by legislation establishing the Department of the Air Force?
answer: June 20, 1941
September 18, 1947
Natural Questions answer: 20 June 1941 (from the Wikipedia article "History of the United States Air Force")