The two companies have teamed up to design and propose the aircraft for the U.S. Army's Future Vertical Lift program.
The service considers Defiant and the Bell V-280 Valor tiltrotor for the FVL initiative.
Randy Rotte, director of FVL at Boeing, told the publication the industry team could push back the maiden flight to 2019 if unexpected issues arise during ground tests that would require additional work.
The companies said they still have time to provide the service with test data needed in the source selection process even if they decide to move the initial flight to next year.
“We'll give them that data, whether it's in December or some later date,“ said Rich Koucheravy, FVL director at Sikorsky.
Sikorsky registered the aircraft as S-100 with the Federal Aviation Administration in August.