This would only work if the 109 was ahead of the 17 when the shot was fired.

It will work fine as long as the two maintain the relative 50 yards
distance between them. - one following the other. Or is Rich S.
description of the motion muddled? I took "crossing behind the B-17" as
simply crossing the line of sight slightly and not a perpendicular
ground path. Normally a 109 would follow a B-17 to attack.