- experimental vehicles
- prototypes
- record breakers (if specially modified)
- flying laboratories
- one of a kind
Examples (some qualify by in more than one category) :
Experimental:
XB-35, X-29, X-47B, Su-47, Maus tank
Ptototypes
YB-49, YF-23, Sukhoi T-50, Concorde G-BSST, A-380 and B-787 in "First Flight" livery
Record breakers
TB-1 "Strana Sovetov", Bloodhound SSC, Ruthan Voyager
SR-71 is an example that DOES NOT qualify as no special modifications were made
Flying Laboratories
NB-52, Tu-144LL
One of a kind
An-225, Sukhoi T-4, Wright Flyer
Worth trying ?