This is a Notable Operational milestone.
The first flight was in conventional flight mode. Pilots were Vought CTP John Konrad and Senior Experimental Test Pilot Stuart Madison. The aircraft achieved a first vertical flight on Dec 29, 1964 (see LTV advert in Aviation Week & Space Technology, January 25, 1965, Page 27). The aircraft subsequently completed a complete transition from vertical take-off to forward flight and vertical landing on January 11, 1965.
Source: 'XC-142 Makes Conventional First Flight', Aviation Week & Space Technology, October 12, 1964, Page 18