The only way to be assured fix wing assignment in the military is to join an ANG unit in your state that flys a fixed wing a/c. Everybody else doesn't find out until they are already in primary. Do you feel lucky?

USA: Almost all rotors
USCG: Mostly rotors
USMC: Lots of rotors
USN: Plenty of rotors
USAF: Not as many rotors

If your ultimate goal is airlines, it will be tough to get back to that after 10 years in helos. The only real save is to pick up a stint as a fixed wing IP.