"The 588th was the most highly decorated female unit in that force, flying 30,000 missions over the course of four years -- and dropping, in total, 23,000 tons of bombs on invading German armies. Its members, who ranged in age from 17 to 26, flew primarily at night, making do with planes that were -- per their plywood-and-canvas construction -- generally reserved for training and crop-dusting. They often operated in stealth mode, idling their engines as they neared their targets and then gliding their way to their bomb release points. As a result, their planes made little more than soft "whooshing" noises as they flew by.
Those noises reminded the Germans, apparently, of the sound of a witch's broomstick. So the Nazis began calling the female fighter pilots Nachthexen: "night witches." They were loathed. And they were feared. Any German pilot who downed a "witch" was automatically awarded an Iron Cross."
Link