Without discouraging further discussion, this question has been asked a few times in the past, for example[here] (https://www.reddit.com/r/AskHistorians/comments/8lxmkx/during_wwii_specifically_were_there_any/?utm_source=reddit-android) with an answer by u_Bigglesworth_ and a link from u/searocksandtrees referencing an even older answer.
In his authoritative work on small arms, Hatcher's Notebook, Major Julian S. Hatcher wrote of experiments with firing various types of weapons directly upwards in order to determine the hazard to ground troops from falling bullets (which could come from very high angle machine gun fire, bomber defensive fires, or the like). The short version is that smaller bullets, such as those fired by the .303 machine guns fitted to the Lancaster and other British bombers, would strike the ground at approximately 300 feet per second. This is sufficient to leave a mark approximately 1/16" deep in soft pine or shallowly dent thin metal. This would probably hurt like the dickens, but is unlikely to kill a unprotected adult human.
Larger bullets, such as those fired by the 0.50" heavy machine guns fitted to American bombers (and later some British aircraft), strike the earth at ~500 feet per second. Given the mass of one of these bullets (~718 grains), that velocity is probably sufficient to cause serious injury.
The effects of wind currents would indeed cause a large amount of scattering, with the bullets coming down relatively unpredictably over a wide area. Additionally, the bullets may strike base-first, point-first, or tumble in sideways making their terminal effects difficult to predict. In any event, it seems safe to say that heavy machine gun bullets would be dangerous for unprotected persons.
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It depends on the type of weapon used, the cartridge fired, wind speed on that day, and the distance between the weapon fired and impact. Many bombers on both the Allied and Axis powers used high-caliber cartridges, which are capable retaining enough energy to kill or maim upon impact at a distance of several miles. Bombing run altitude would have played a major part in this. An excerpt from Airforcemag says "Eighth Air Force in Great Britain put 31.8 percent of its bombs within 1,000 feet of the aim point from an average altitude of 21,000 feet. Fifteenth Air Force in Italy averaged 30.78 percent of its bombs within 1,000 feet from 20,500 feet." Assuming that the rounds were fired during a low altitude run, using the 303. Brit cartridge that was standard in their machine guns, it's reasonable to say that it would have enough energy to maim or potentially kill any unlucky bystander on a direct hit. I feel it's also important to mention that at that height, the impact of the round would be far from where it was originally aimed, since 303. Brit isn't the flattest shooting of rounds. Here are the ballistics data for the 303 Brit. It's ballistically similar to other standard infantry cartridges of the era, such as 30-06, 7.62x54r, 8mm Mauser, etc. etc. It's also important to mention that many standard infantry rifles of that era had sights with elevation markings for up to 1600 yards, for indirect volley fire. Higher caliber weapons, such as the Browning Machine Gun used in American bombers used the 50 BMG cartridge, which retains much more energy over a longer distance. Other things that are impossible to account for are the direction that the weapon was fired. Fighter planes usually approached from the rear of a bomber, and it's reasonable to assume that many such rounds were fired laterally. Upon firing, those rounds would travel a much farther distance laterally than they would if fired downward, and lose much of their energy by the time it hit the ground.