From: Seoul, Korea
Against the AI I'd say air forces are a handicap to the AI and you shouldn't use them. In PBEM it might be a fun element.
I noticed the AI builds a lot of AA units, which don't seem to be good for anything else, even against tanks. I also didn't see the AI build any aircraft.
Once my tech became reasonable, I built some helos to go after big alien critters, not the normal AI opponents, since the normal opponents had quite a bit of AA by them. Still, ground attack helos didn't do much to the critters. I didn't bother with airplanes since the opportunity to use them would have been minimal what with having to be based at a city with an airfield.
For the aircraft design process I'd start with a list of maximum parameters for each type (e.g. 2000-pound bombload, range of five hexes, speed of 200 knots, cargo of 2000 pounds, etc.) and you'd pick one that's the most important to you, say the bombload of 2000 pounds, and then it would show you what was still achievable with the other parameters (e.g. range of three hexes, 170-knot speed, no cargo, etc.) and you'd narrow it down with each choice.
Beer, because barley makes lousy bread.