From the Oxford English Dictionary at OED:
proof of concept n. evidence (usually deriving from an experiment or pilot project) demonstrating that a design concept, business idea, etc., is feasible; a piece of such evidence; freq. attrib.
1994 San Antonio Business Jrnl. (Nexis) 19 Aug. a4 "We built this center as a proof of concept."
2001 Today's Pilot Feb. 51/1 "Although the GA-200 is a totally new aircraft, some heavily modified Pawnees were used as proof-of-concept aircraft."
So is it both a noun and an adjective?
I wrote in a document "(...) successfully evaluated through a proof of concept prototype." and now I'm wondering whether I should change it into "(...) successfully evaluated through a proof of concept.". I left the dashes out in both cases.