The B-17 was a low-wing monoplane that combined aerodynamic features of the XB-15 giant bomber, still in the design stage, 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." The B-17E, the first mass-produced model Flying Fortress, carried nine machine guns and a 4,000-pound bomb load. It was several tons heavier than the prototypes and bristled with armament. It was the first Boeing airplane with the distinctive -- and enormous -- tail for improved control and stability during high-altitude bombing. Each version was more heavily armed.