I don't know about the other services, but in the Navy a senior enlisted man with critical skills or experience can become a Warrant Officer if he chooses, a rank that is outside but parallel to the usual officer grades. WOs are sailors that are awarded officer rank without a regular commission or coming up the ranks or having attended Officer Candidate School or graduated from the Naval Academy. This is usually done to reward individuals with valuable, highly specialized expertise to keep them in the service. Warrant Officers are rare (I never met one).

In the chain of command, the WOs are senior to the highest level of enlisted man (E-9, or Master Chief) but junior to even the lowliest Ensign (O-1). Its sort of a limbo rank, they are accorded respect because of their expertise, but they do not hold command positions outside their own technical specialty.

When I left active service in the fleet, I was an E-4 (third class petty officer or QM3) although I had passed my exams for E-5 (QM2), but I was not promoted unless I "shipped over" (Re-enlisted). However, when I reported to my hometown reserve unit for standby duty I was allowed to sew on my "crow". I am told that for a reservist to make E-5 after only 13 months of sea duty was highly unprecedented, but it was wartime...

When the Captain gave me my shipping over speech I was offered the rank, a $3000 cash bonus, and my own command, a Patrol Boat River in the brown water Navy, just like the PBR in "Apocalypse Now". Gunner's Mates, Boatswain's Mates and Quartermasters were the only enlisted ratings eligible for that post. I politely declined.