If you wanted to stretch you could say its roots could be traced back to the balloons both sides used in the War Between the States. I think we could safely go back to, I think, 1905 when the Army bought its first airplane from the Wrights. As a separate organization from the Army, it's very definitely 1947. As an established and permanent part of the American armed forces, to include Army and Air Force, I think 1905 could be defended.