So let's say that ingame they are even 10x better than real life. Very few guys are going to have the patience to fire that many rounds to take a level bomber down.
In AH, most buffs fly considerably lower than in real life, and use very suicidal tactics. A formation B-17's that is passing over the town at 'realistic' 25,000 feet will be very, very difficult to hit from the airfield.
Quick & rough computation:
If the town is 5k yards away from the field and the bombers are at 24K feet... if directly over the town the icon range should be about 9.4k The rounds take about 21 seconds to get there, during that time a formation of B-17s will travel something like 2500 yards = ~ 100 plane lengths. And of course that 21 seconds means you would shoot when the bombers icon range would display a different readout than 9.4k. Also you would have to aim for a point way in front of the town, as the bomb release point is not directly over it and bombers often change course right after dropping the bombs.
Lots of variables, many guesstimates and enormous room for error.
But stil: Given the laws of probability it's just a matter of time until such a thing happens - and we can already bet on how long it will take for the first "ridiciously overmodeled crap" whine to show up