As World War II intensified, the Allied bombers needed additional armament and armor. 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 new version was more heavily armed with the G model representing the maximum defensive fire- 11 machine guns. The Fortresses were legendary for their ability to stay in the air after taking brutal poundings. Boeing plants built a total of 6,981 B-17s in various models, and another 5,745 were built under a nationwide collaborative effort by Douglas and Lockheed (Vega).