Well then factually correct it. Please.
Based on my ASVAB and basic physics/nuclear testing results, I was permitted to select my general field (nuclear) and specific NEC (3383, surface vessel reactor operator) for the US Navy, back in '91. Nuclear field was specified in my enlistment contract before signing, and reactor operator was contracted in writing for me in boot camp after further testing. As Balog stated, all guarantees are subject to successful completion of .mil training school(s) - in my case, ET (A) school, Naval Nuclear Power School, and Naval Nuclear Prototype training, a curriculum totalling approximately 2 years (thus requiring a 2-year contract extension for active duty which came out of the Individual Ready Reserve requirement - instead of 4yrs active/4yrs IRR, it was 6/2).
The .mil allows EVERYONE to put in for whatever they want. Needs of the service do come into play, but if you qualify for advanced training and technical service, you'll almost certainly find that the service needs you in that position rather than as an infantryman or cook. And if you don't get it in writing before signing, you deserve what you get.