U.S. General Hawk Carlisle, who heads Air Combat Command, will announce at a news conference at 1:00 p.m. EST (1700 GMT) that the Air Force has an “initial operational capability” of the F-35 jets, said the sources, who were not authorized to speak publicly.

The decision marks another big milestone for the $379 billion program, the Pentagon’s largest weapons project. The Air Force follows the U.S. Marine Corps, which declared a first squadron of F-35s ready for combat in July 2015.

Officials say the F-35 will give the U.S. military the ability to detect enemy aircraft and other threats far beyond current ranges, allowing the jets to strike targets and disappear long before they are detected.

The U.S. Air Force plans to buy a total of 1,763 F-35A conventional takeoff and landing jets in coming years and will operate the largest F-35 fleet in the world.

Carlisle told reporters last month he was “very confident” in the new stealthy warplane, and lauded its performance during a practice deployment to a second air base, where the jets successfully completed all 88 missions planned.

The Pentagon’s F-35 program office on Monday announced that the F-35A had launched an AIM-9X air-to-air missile to hit and “kill” a target simulating a drone over a military test range off the California coast on July 28.

“After launch, the missile successfully acquired the target and followed an intercept flight profile before destroying the drone, achieving the first F-35 air-to-air kill or ‘Boola Boola,’ which is the traditional radio call made when a pilot shoots down a drone,” the program office said.

The same F-35 pilot who hit the drone also used an internally carried AIM-120C missile against another target drone that was beyond visual range, telling it to self-destruct right before impact, the program office said in a statement.

“It’s been said you don’t really have a fighter until you can actually hit a target and we crossed that threshold with the first air-to-air weapon delivery of an AIM-9X,” said Air Force test pilot Major Raven LeClair.

Lockheed is building three models of the F-35 Lightning II for the U.S. military and 11 countries that have already ordered the jets: Britain, Australia, Canada, Norway, Italy, Turkey, Denmark, the Netherlands, Israel, South Korea and Japan.

The program, first launched in 2001, has made strides in recent years after huge cost overruns and technical problems that sent the project cost up by nearly 70%.

Industry and U.S. defense officials say they are working hard to continue driving down the cost of the new warplanes to around $85 million per plane by 2019, as well as the cost of operating the jets.