OV-099 Challenger was renumbered from STA-099 (Structural Test Article), it was not originally built to be flown.

OV-099 as a number actually doesn't make sense, because the numbering scheme (OV-XYY) in full reads: Orbiter Vehicle, Series X, Vehicle YY.

Series 1 is the original (and only) line of flightworthy Space Shuttle Orbiters including Enterprise, Vehicle number is given in sequence within a series starting from 01.

So OV-101 (Enterprise's) reads Orbiter Vehicle, Series 1, Vehicle 01. OV-102 (Columbia's) reads likewise Vehicle 02, and so on.

OV-099 (Challenger's) reads Orbiter Vehicle, Series 0, Vehicle 99 which makes absolutely no sense.

If we really want to get particular there was OV-098, Pathfinder, though being made of wood it obviously was never meant for more than fitment testing. Oddly though it did get the OV designation, not the STA designation.