From the Document: "When it was established as a separate service in 1947, the U.S. Air Force (USAF) was to be 'organized, trained, and equipped primarily for prompt and sustained offensive and defensive air operations ... necessary for the effective prosecution of war except as otherwise assigned.' That statutory language remains almost identical today. Similarly, although the words used to describe its core missions have changed, space and cyberspace joined air as operational domains, and the means used to carry them out have evolved with technology, the USAF's missions themselves have remained remarkably constant."
CRS In Focus, IF10547
Congressional Research Service: https://crsreports.congress.gov/