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 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, and for having suffered the greatest number of losses of any heavy bomb group in World War II.