How about having squadrons or groups of aircraft function as some sort of umbrella. If they have a lot of airpower, the umbrella extens up to the maximum range. If the group has a lot of mudmovers, enemy units under the umbrella suffer more damage per turn, or frienly units get a bigger bonus in ground attacts..
I think that one must make a clear difference between all the aspects of air-combat. 3 aspects come to mind:
1 Air control. Shooting down other aircraft, be they fighters or bombers, and escorting one's own bombers.
2 Tactical bombing. Ground forces are supported by medium bombers or mudmovers. Tactical bombers are probably not as vunderable to lack of good aircover than strategic bombers.
3 Strategic bombing. Bombing enemy cities, factories or transport lines. This probably requires direct player control, while the other aspects require only stationing of aircraft.
I can be somewhat more specific about the "umbrella" aproach. Fighters, strategic and tactical bombers each make their own umbrella's. These umbrella's perform seperately from eachother. Off course, fighters still need to protect the bombers, so the fighterumbrella should cover the theatres of operations of the bombers quite well.
Another important aspect is the intensity. To put it simply, more fighters mean more enemy aircraft shot down and less bombers lost due to enemy fighters. The umbrella indicates where aircraft perform their missions, while intensity indicates how often and how well they do their missions. Intensity means strengh and a greater distance means less intensity.
Final point/example. When building and managing fighters one has to look at 2 things. Fighting capability and range. The maximum range indicates the maximum size of the umbrella. So if no enemy fighters would impede our fighter, it could fly all the way to the edge of the umbrella to perform a mission. Fighting capability and numbers go hand in hand, together they make up for the intensity. Crap fighters don't shoot down other fighters well, however if you have loads of them, they might protect your bombers far enough to perform a mid range mission.