The Boeing B-17 Flying Fortress is an American four-engined heavy bomber developed in the mid-1930s for the United States Army Air Corps (USAAC). A fast and high-flying bomber used primarily in the European Theater of Operations, the B-17 dropped more bombs than any other aircraft during the Second World War. It is the third-most produced bomber in history, behind the American Consolidated B-24 Liberator and the German Junkers Ju 88.

In a USAAC competition, Boeing prototype Model 299/XB-17 outperformed two other entries but crashed, losing the initial 200-bomber contract to the Douglas B-18 Bolo. The Air Corps ordered 13 more B-17s for evaluation, introduced into service in 1938.

The B-17 evolved through numerous design advances and from its inception was promoted as a strategic weapon. It developed a reputation for toughness based on stories and photos of badly damaged B-17s safely returning to base.

The B-17 saw early action in the Pacific War against Japanese shipping and airfields, but was primarily employed by the USAAF in the daylight component of the Allied strategic bombing campaign from UK airfields.