High Quality Content by WIKIPEDIA articles! The Boeing YB-40 Flying Fortress was a modification of the United States B-17 Flying Fortress bomber aircraft, converted to act as a heavily armed escort for other bombers during World War II. At the time of its development, long-range fighter aircraft such as the North American P-51 Mustang were just entering quantity production, and thus were not yet available to accompany bombers all the way from England to Germany and back.