The B-17 Flying Fortress was a low-wing monoplane that combined aerodynamic features of the XB-15 giant bomber, and the Model 247 transport. The B-17 was the first Boeing military aircraft with a flight deck instead of an open cockpit and was armed with bombs and five .30-caliber machine guns mounted in clear blisters. Each version of the B-17 was more heavily armed. Described by General H. H. Hap Arnold, as the backbone of our worldwide aerial offensive, the B-17 Flying Fortress served in every World War II combat zone. Boeing, Douglas, and Lockheed Vega produced 12,731 B-17s, and the four-engine bombers became legendary for their ability to stay in the air after taking brutal poundings.