This seems backward at first, but Wald realized his data came from bombers that survived. That is, the British were only able to analyze the bombers that returned to England; those that were shot down over enemy territory were not part of their sample. These bombers’ wounds showed where they could afford to be hit. Said another way, the undamaged areas on the survivors showed where the lost planes must have been hit because the planes hit in those areas did not return from their missions.
Wald assumed that the bullets were fired randomly, that no one could accurately aim for a particular part of the bomber. Instead they aimed in the general direction of the plane and sometimes got lucky. So, for example, if Wald saw that more bombers in his sample had bullet holes in the middle of the wings, he did not conclude that Nazis liked to aim for the middle of wings. He assumed that there must have been about as many bombers with bullet holes in every other part of the plane but that those with holes elsewhere were not part of his sample because they had been shot down.
For several months after the US entry into World War II, Americans feared air raids or even invasions of the continental United States by Germany and Japan. An article in the March 2, 1942 issue of Life magazine suggested several possible invasion routes that Axis forces might take. These include a Germany landing at Norfolk, Virginia supported by fifth columnists hidden in the US. At the link, you can view maps of a few other fanciful scenarios.
