Boeing B-17N Flying Fortress
The Boeing B-17 Flying Fortress first flew in 1935. Because of the mounting tensions in Europe and in the Pacific the USAAC wanted to have a heavy bomber capable of greater speed and range than the Martin B-10 then in use. The first impressions were very good and in 1939 several hundred were ordered. The number of B-17s used by the USAAC grew quickly with 500 further aircraft ordered in 1940 and 700 in 1941.
When the German - Russian cease fire was signed however at the end of 1941, the tensions in the world decreased. At first US attention shifted to the Pacific and the Japanese but after the big Japanese defeats in China the threat diminished significantly. Spending for the US armed forces was decreased and new aircraft programmes were cancelled. This included the far more advanced Boeing Superfortress which was cancelled before the first prototype was finished.
The USA chose a more and more isolationist route and kept her distance to the conflicts in Europe between Germany and Great Britain. The tensions in the world decreased after this and a long period of detente between the USA and Germany started at the end of the forties.
Arms spending dwindled almost to zero in the USA and the armed forces had to keep their existing material working as no new programmes were initiated. This left the B-17 as the main US bomber for many decades. The existing airframes were upgraded many times with new engines and better armament. The ultimate model of the B-17 was the B-17N which appeared in 1971. The B-17N consisted of upgraded B-17K and B-17M airframes with Wright R-1824-243 engines giving 1800hp on take-off.
The B-17N was in service until 1983 when it was finally replaced by a new strategic bomber, the North American B-27 Lancer.