I saw a television show on the History Channel recently that talked about the Norden bombsight. As part of the show, they took up a B-25 with a Norden in it and had two bombardiers try to hit a cross painted on the ground. I forget the altitude they were flying at, but I think it was 12k or so -- it certainly was not 25k. Also, the bombardiers didn't get much practice with the Norden before using it, which you could tell was a big issue (as the bombardiers were saying that they were having some trouble getting it all set up while on the bomb run). They had one guy who flew in WWII and one who flew in Korea using the Norden. Although neither hit directly in the center of the cross, both got decently close, with one of the guys putting a bomb about 100 ft from the center.
My guess is that it would have been possible for bombers to place single bombs fairly accurately. However, there are many things that could hamper that ability. Targets generally would take many bombs to be destroyed -- especially fortified targets. So, more bombs or more bombers were needed. Also, once one bomber drops, there is dust and smoke that obscures the target, so there is more guesswork on deciding where to drop if you are a following bomber, and again more bombers are then needed. I don't think wind and air density were the main problems, though, as the Norden was there to calculate bomb trajectories taking into account such things.
If in Aces High we had (1) to contend with much poorer visibility (due to clouds over target that you couldn't see through, not just haze that you can still see through; and due to large amounts of smoke or dust, enough to obscure the target thoroughly) and (2) contend with ground landscapes that didn't make it as easy to spot and to know exactly where a particular building or AA gun is, I think you'd see people needing to put down sticks of bombs instead of one bomb for each target.