A new aircraft designed by Aurora Flight Sciences to experiment with active flow control technology has been allocated the designation X-65 by the
Defense Department: aviationweek.com...
Given that the F-16 Variable In-Flight Stability Test Aircraft was previously the most recent experimental aircraft to receive an
X-for-Research/Experimental design number, being designated X-62 in 2021, it's quite odd that X-63 and X-64 were skipped, but the Aviation Week and
Space Technology article considers it possible that X-63 and X-64 have been allocated to other known experimental aircraft projects, such as DARPA’s
Liberty Lifter aand the Speed and Runway Independent Technologies (Sprint) programs.