Lieutenant General Jon Davis, the deputy commandant for aviation, told Reuters he was keeping close tabs on 13 items required to approve combat use of the jets. He said he remained optimistic that the service would meet its July target date for the milestone.
"We're targeting July 15," Davis said. "We're hell-bent on getting this airplane into service correctly. We're not going to declare IOC unless they're ready and they can do all the things that they've said they can do."
If the Marines Corps grants its approval by then, it would be the first military service to certify an "initial operational capability" of the F-35, the Pentagon's biggest weapons program ever, costing nearly $391 billion.
Air Force Lieutenant General Chris Bogdan, who runs the F-35 program for the Pentagon, has said he is pushing for the program to complete all key requirements to certify IOC by July 1.
The stealthy fighter – built by Lockheed Martin – was designed to replace a dozen planes currently in use by the US military. Britain, Japan, South Korea, and Israel are among a number of American allies that have already placed orders for the jet.
Lieutenant General Davis said the Marines planned to deploy the first F-35 squadron to Japan in January 2017, but it could respond to crises around the world as soon as the IOC declaration was made.
After deficiencies were found in several of the jet’s systems, Davis said he planned to carry out an unprecedented "operational readiness inspection" before approving the first squadron for combat use. That includes academic tests for officers and enlisted personnel, simulator flights and test flights.
"I want to prove to people that we are very serious about this, and that we have no intention of putting an airplane in a combat theater that's not ready to go," he said, adding that the inspection would likely take "a couple of weeks."
By July, the Marines will have more than 50 trained F-35 pilots and more than 400 maintenance personnel, a spokesman said.