The Boeing B-17 Flying Fortress, which was used by the US Air Force for the duration of World War II, is perhaps the most famous device used by the Americans in the war.The B-17 was designed in 1934 as a response to a request made in May by the US Air Corps for a unit that had to meet these requirements: multi-engine, capable of carrying at least 2,000 pounds (907 kg) of bombs on a minimum distance of 1,020 miles (1,640 km) at a speed of 200 miles/h (320 km/h) and beyond. Although Boeing was working drawings of another, more heavy bomber, the model 294 or XB 15, the design and implementation of new aircraft proceeded rapidly and the prototype model 299, as it was called by Boeing, rose for the first time in flight in July 1935.This prototype is sometimes called, erroneously, XB-17.Power was provided by four radial engines Pratt & Whitney R-1690 Hornet that erogavano 750 HP each; The defensive armament consisted of five machine guns single 0.30 inch (7.62 mm), while the offensive load could reach 4,800 pounds (2,177 kg). Although the prototype was destroyed in an accident, his performance was so encouraging that the USAAC ordered the construction of 14 YB-17, later renamed Y1B-17. The first of this group was delivered in December 1936 and was equipped with radial engines Wright GR 1820 of 930 HP. The first 13 aircraft of this group were all the same, while the fourteenth was the only Y1B-17A, equipped with turbo-superchargers for engines, which allowed to reach a maximum speed of 311 miles/h (500 km/h) and a maximum altitude of operation of 30,000 feet (9,144 m); these variants were then incorporated into all models B-17 that followed.