The 91st Bomb Group (Heavy) was an air combat unit of the United States Army Air Forces during the Second World War. Classified as a heavy bombardment group, the 91st operated Boeing B-17 Flying Fortress aircraft and was known unofficially as "The Ragged Irregulars" or as "Wray's Ragged Irregulars", after the commander who took the group to England. During its service in World War II the unit consisted of the 322nd, 323rd, 324th, and 401st Bomb Squadrons. The 91st Bomb Group is most noted as the unit in which the bomber Memphis Belle flew (in the 324th Bomb Squadron), and for having suffered the greatest number of losses of any heavy bomb group in World War II.

The Memphis Belle is a Boeing B-17F Flying Fortress used during the Second World War that inspired the making of two motion pictures: a 1944 documentary film, Memphis Belle: A Story of a Flying Fortress and the 1990 Hollywood feature film, Memphis Belle. It was one of the first United States Army Air Forces (USAAF) B-17 heavy bombers to complete 25 combat missions, after which the aircrew returned with the bomber to the United States to sell war bonds.