With a 1,000 B-17s, each with 10, 50 cal machine guns potentially firing, bullets fired must eventually reach the ground.

That’s a very interesting proposition! I’d always assumed that after a few thousand metres any bullet fired from a B-17 (say) would be relatively harmless but it seems that the terminal velocity of a 50 calibre bullet is about 135 metres-per-second (450 feet-per-second); that may not sound fast compared to the muzzle velocity of 2900 feet-per-second but it is still about 300 miles-per-hour!

A 50 calibre round, even if it was fired upwards from a B-17 flying at 24,000 feet, would still be lethal when it reached the ground!