The B-17 was a four-engine heavy bomber developed in the 1930s for the U.S. Army Air Corps and was the first aircraft assigned to Rapid City Army Airfield, which eventually became Ellsworth Air Force Base. The last model, the B-17G, housed 13, .50-caliber machine guns to help in fending off fighter attacks. There are 53 displayable B-17s left in the world.