My DS, USMMA 2013, is a Navy pilot, so I can speak from his experience. You can apply for a flight spot for any of the five services out of USMMA. He applied for USN and USMC, and got pilot offers for both. He spent his sea year internship at NAS Key West and had a blast. He has friends who got flight spots in CG, AF, and Army also. All the initial physical screening is done before you commit, so if you fail there, you can reassess your choices. Of course you can still get bumped out medically during your full flight physical, but you can at least make it through the initial DQ phase before committing. DS had a classmate who wanted a pilot slot, but was only offered NFO. He elected to sail on his license rather than go AD. Nearly everyone who wants an aviation slot out of USMMA gets it, but not always your first choice branch and not always pilot vs. NFO.
Be advised though that before you select USMMA in your quest to be a pilot, you should have an interest in learning about things maritime. It is primarily a maritime school. If you do not have that interest in addition to your ambition of being a pilot, do not take the USMMA route. The curriculum is tough, the fields of study are very limited, and you will likely fail. For those who do have that dual interest, it is a great option. My DS had no regrets. Good luck!