First you'll need to find the bit where the AI decides on what air mission to do. You'll probably want to get something that judges how many aircraft are needed for defence (CvCity->getNumAirDefenders() or something similar), and then use any excess aircraft, or maybe a certain percentage of the defending aircraft which is either, dependant on how the war is going (very general... might be hard to do), or fixed, to conduct the mission.
Then you'll have to find a suitable target. Best ones would be near cities you have bombers and land troops in range of. There might already be some functoins in place which you may be able to use to help with this - I'm not sure.
You'll also have to fix the unit order in there somewhere so that the bombers don't do their bombing runs before the fighters have tried to clear the skies.... but the bombers need to do their bombing runs before the units move in.