In response for the Army’s request for a large, multiengine bomber, the B-17 (Model 299) prototype, financed entirely by Boeing, went from design board to flight test in less than 12 months. The B-17 was a low-wing monoplane that combined aerodynamic features of the XB-17 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. In the Pacific, the planes earned a deadly reputation with the Japanese, who dubbed them four-engine fighters. The Fortresses were also legendary for their ability to stay in the air after taking brutal poundings. They sometimes limped back to their bases with large chunks of the fuselage shot off.