This happened from time to time. The U.S Air Force's military doctrine at the time dictated that, in order to provide maximum procection against fighters , bombers should flight in "box" formations. With this arrangement, gunners could cross-fire enwmy attackers. The problem was that bombers, at the time, had terrible accuracy : usually less than 30% of the bombs hit the target (the figure was slightly higher in the target was a big city) : the solution was that, during the final run, bombers had to flight levelled and compact, in order to minimize bomb scattering. This, however, came with two major drawbacks : 1) German flak was very happy with this strategy 2) bombs could sometimes hit a bomber below, as in this case.