My fiancee cross-commissioned from USMA to the Marine Corps. In short, the reason he's flying jets in the Marine Corps and not helicopters in the Army was about 5% his performance and 95% luck/timing. It was not easy and he had to jump through many hoops for it to happen (do USNA Leatherneck and some other training stuff with the Marines), culminating in him putting together this ridiculous packet that ended up being about 80 pages long. Five or six other people from his class tried to go Marines as well and only two total got it. He didn't know until April or so before commissioning what service he was going to commision into.
It's a fine goal, but you need to be fully prepared for:
-It not getting approved and you serve in the Army
-It getting approved, but you don't get the community you want in the Air Force and spend five years handing out towels as an MWR officer. This is especially true for you since you're looking at an extremely competitive job in the Air Force
If either of those things happen, both of which are far more likely than getting exactly what you want, you need to be ready to go and not be "that guy" who's bitter about not getting his dream job. If you're still set on going for it, start demonstrating interest from the start and seek out other Cadets who are trying to cross-commission as well to get a look at what their request packets look like.
Out of curiousity though, why try to go Air Force when you can do plenty of cool-guy special things in the Army?
