Constant flight speeds made bombers predictable targets—“sitting ducks”—not unlike fixed townships and cities, and in many ways the evolution of these planes paralleled the evolution of walled fortifications and castles.26 Bombers did not attack rival bombers, like fighters attacked fighters. Instead, their survival depended upon defense, thwarting the advances of enemy fighters. In these confrontations, exposed planes were dead planes, so rotatable, protruding turrets with machine guns were added to the top and bottom, and guns were added to the nose, tail, and flanks. As with castles, the idea was to leave no flank undefended, and bombers soon carried guns and crew sufficient to provide covering fire to all sides of the craft. Even the names of these planes reflected the defensive logic of their design, such as the B-17 “flying fortress,” or the B-29 “superfortress.”

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 550