I can speak with some authority on this subject - I graduated from Air Force Pilot Training in 1992 and did my last assignment as an instructor at a UPT base, teaching students how to fly.
First - the altitude chamber training is done very early in the whole process (around week 3 of a 52-week course). This is training only, and has no bearing at all on what sort of airplane you will fly. There are the very rare occasions where someone experiences previously unknown physiological symptoms which might cause them to not be able to proceed to the flight phase, but these are so rare that they are almost not worth mentioning.
So, the altitude chamber is as **LSLGuy ** and flyboy88 have described it.
As to who gets to fly what jet, it’s a complex process. The Air Force has gone to a tracked system (similar to what the Navy has been using for a long time). Note - their is some cross-service training, so a random Air Force guy might do his first stint with the Navy flying T-34s, and some Navy guys come fly T-6s (previously T-37s) with the Air Force.
The first flight phase is now done in T-6s, and the students are graded on academic scores, emergency procedure evaluations, simulator training, daily flights, checkrides and a catch-all “military bearing”. Each segment is weighted differently, (ie a screw-up on a checkride will cost you dearly, but a screw-up on a regular daily flight won’t hurt you as much). At the end of the phase all the scores are added up and the students are ranked.
This is when the first “needs of the Air Force” comes into play - the students can select either the T-38 track (leading to fighters and bombers) or the T-1 track (leading to heavies). Of course the students are limited by how many of each track are available. I have seen it both ways - more students want the fighter track but can’t get it, or more students want the heavy track but can’t get it. in the end, they choose their track and proceed onto the next phase of training, flown in the T-38 or the T-1 (or sometimes with the Navy in the T-44 if they are going to fly C-130s).
The advanced phase works much like the primary phase, but with more specialized training. The students are graded in all the same areas and then ranked. About 6 weeks before graduation a list comes down of what airplanes will be available for the class (since there will be classes graduating on the same day at all three UPT bases, the list covers all three). The students then rank-order their preferred aircraft. The higher you finish, obviously, the better your chance of getting what you want.
Of course the aircraft vary wildly depending on timing. When I graduated we had one F-16 for (at the time) five bases, and half of the people didn’t even get a flying job - they were “banked” and went to fly a desk for three years. We also didn’t do the tracked system, so we were all coming from T-38s. It’s not like that now, but one class can get a ton of “desirable” aircraft (say C-130s to Germany, or F-15Es to England) while the next can be saddled with “undesirable” aircraft (KC-135s to Grand Forks, or B-52s to Minot). I put desirable and undesirable in quotes because beauty is in the eye of the beholder - some guys want nothing more than to fly a B-52 and wouldn’t ever consider getting in an F-15, just as some want to fly C-130s and would never get near a C-5.
The squadron commanders then go down the list and see what each student wants and whether it is available. With, say, two C-17s available and the first two students choosing them the guy (or gal) who finished #3 will not be getting a C-17. If he wanted one, tough.
The assignments are handed out on “drop night” (as in assignment drop). This is done on a Friday at the Officer’s Club, and is always a lot of fun. Well, fun for most, misery for some but always a memorable night (or not, depending on how many beers they drink).
Sorry for the long post, but the assignment of pilots to aircraft is a complex process that involves numerous checks and evaluations of the student but also depends heavily on what the military needs at that time.