The U.S. Marine Corps on Friday said the F-35 Joint Strike Fighter is finally ready for combat, a milestone for the world's most expensive weapons program and one that is likely to shift debate to the jet's capabilities.

The announcement by Marine Corps Commandant Gen. Joseph Dunford comes almost 14 years after Lockheed Martin Corp. beat out Boeing Co. for what's become the world's priciest military contract, with the Pentagon planning to spend almost $400 billion to develop and buy more than 2,400 jets.

The declaration of initial operating capability, or IOC, means the first squadron, comprising 10 of the F-35B model jets based at Yuma Air Force Base in Arizona, can now be called up to fight. There was a final assessment of their operation, maintenance and training earlier this month. The first aircraft aren't scheduled to be deployed overseas until 2017 when a squadron is due to be stationed in Japan.

The F-35 is entering the fray more than four years late, and costs have spiraled to twice their original estimate, which has made it the subject of intense debate and criticism. The Marines' move is expected to push supporters and critics alike to focus on the jet's capabilities rather than just its cost.

Mandy Smithberger, a director at the Project on Government Oversight, said the Marines were determined to meet their July target even if the plane wasn't fully ready. "We don't think this is a genuine IOC," said Ms. Smithberger, whose watchdog group has long been a critic of the F-35 program.

Ms. Smithberger said the arrival of the first combat-ready jets will allow it to be compared more realistically to the planes it is due to replace, including the F-16 and A-10 Warthog. "Does the F-35 do this as well or better than what we're throwing to the boneyard?" she said.

Critics have pointed to the F-35's mixed performance in air-to-air combat tests with other jets, though the Pentagon has said this isn't its main role. There are also concerns that the absence of a gun, which won't be ready until 2017, limits its ability to protect ground troops.

The Marines said the F-35B will enter service without some other features it had wanted from the start, including the ability for the jets' sensors to communicate properly with other planes. A more advanced pilot helmet with improved night vision also isn't ready. However, the Marines said the F-35 is still a huge improvement on its existing, aging jet fleet.

"I'm very confident I could send them to any place in the world," Lt. Gen. Jon Davis, the Marines' deputy commandant for aviation, told reporters earlier this week. The F-35Bs can carry missiles and bombs, and he said the gun wasn't a problem as other jets will provide that type of support for ground troops until all of the F-35's planned weapons are available.

The F-35B model is the most expensive of three versions of the jet being developed, with the latest models costing around $134 million each, and can take off and land from short runways or vertically like a helicopter.

The Pentagon expects to pay almost $400 billion developing and buying the jets over the next 30 years to replace hundreds of older planes. The Air Force expects to declare its first batch of F-35s combat ready next year, with the Navy following in 2018 or 2019.

Northrop Grumman Corp, BAE Systems PLC, the Pratt & Whitney unit of United Technologies Corp. and Rolls-Royce PLC are also among the largest of more than 1,200 suppliers world-wide for the F-35 program.

The F-35's troubled gestation stemmed from the decision to have concurrent development and production, forcing Lockheed and its partners to go back and redesign parts of the plane when problems were uncovered. Development isn't expected to conclude until 2017.

Write to Doug Cameron at doug.cameron@wsj.com

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