The Boeing B-17 Flying Fortress is a four-engined heavy bomber developed in the 1930s for the United States Air Force. It was a relatively fast, high-flying, long-range bomber with heavy defensive armament, initially at the expense of bigger payloads. With the first prototype flying successfully in July 1935, a reporter at the scene described it as a '15-ton flying fortress' referring to the number of machine guns sticking out. Boeing was quick to capitalize on that and trademarked the name for use. The prototype not only exceeded the USAF requirements but was also faster and had a longer range than the twin-engined competition from Douglas and Martin.