What about distance from the target? In the 1920s the Royal Navy did lots of air-dropping tests with the following results:
1,250 yards distance: hit probability 10%
1,000 yards: 30%
750 yards: 50%
600 yards: 85%
Such attacks were a bit hazardous. In WW2 the length of a tour of duty (30 missions for bombers) was rated at just 3 (three!) missions for torpedo attacks. I don't know if anybody calculated the probability of scoring a hit against that of being killed, but I'd rather be on the ship....
Tony Williams
New book: Rapid Fire - The Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces.
Details on my military gun and ammunition website:
http://www.users.globalnet.co.uk/~autogun/