Williams Air Force Base is a former United States Air Force (USAF) base, located in Mesa, Arizona, and about 30 miles (48 km) southeast of Phoenix. It is a designated Superfund site due to a number of soil and groundwater contaminants. It was active as a training base for both the United States Army Air Forces, as well as the USAF from 1941 until its closure in 1993. Williams was the leading pilot training facility of the USAF, supplying 25% of all pilots.