Officials said the schedule has been moved to June 24, which will mark the first mission of the rocket for the military.
STP-2 will bring 24 satellites into three different orbits around Earth, including projects from the military, the National Oceanic and Atmospheric Administration, NASA and academic institutions.
“We are now looking at no earlier than June 24 while we finish up integrating these satellites and finish our launch operation preparations,“ said Lt. Col. Ryan Rose, chief of small launch and targets division at the Air Force's Space and Missile Systems Center.
The Falcon Heavy rocket for STP-2 will use a new center core booster and two side boosters recovered from an April 11 commercial launch.
The Air Force originally planned to launch the mission in 2015.