The Boeing B-17 Flying Fortress was a four-engine heavy bomber aircraft developed in the 1930s for the then-United States Army Air Corps (USAAC). From its introduction in 1938, the B-17 Flying Fortress evolved through numerous design advances.
The B-17 was primarily employed in the daylight precision strategic bombing campaign of World War II against German industrial and military targets. The B-17 also participated to a lesser extent in the War in the Pacific where it conducted raids against Japanese shipping and airfields.
With a service ceiling greater than any of its Allied contemporaries, the B-17 established itself as an effective weapons system, dropping more bombs than any other U.S. aircraft in World War II. Of the 1.5 million metric tons of bombs dropped on Germany by U.S. aircraft, 640,000 tons were dropped from B-17s.
Source: Wikipedia, "Boeing B-17 Flying Fortress", available under the CC-BY-SA License.
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Aces of the Pacific: The Boeing B-17D Flying Fortress was one of the first B-17 models to enter service with the AAF. Without power turrets, its defensive capacity was limited. It saw heavy service in the Philippines in 1941. Boeing's B-17E Flying Fortress was a strong, sturdy and heavily armed bomber. The B-17E replaced the B-17D by mid-1942. It saw excellent service in the South Pacific before it was replaced by the B-24 Liberator.
Secret Weapons of the Luftwaffe: Since Luftwaffe fighter pilots were becoming increasingly successful at damaging and downing B-17s by frontal attacks, the B-17G was introduced in 1943. It had a twin gun turret in the nose, which was developed to bring more machine guns to bear on the enemy fighters. Later models of the B-17G also had staggered waist gun positions, to keep the waist gunners from bumping into each other while firing, plus a redesigned rear turret with larger windows. These were the only major improvements, and the B-17G was virtually identical in performance to the B-17F.