The Marine Corps, the ascetic tribe of “Devil Dogs” that prides itself on being “the first to fight,” is getting a new weapon, announcing Friday that its version of the F-35 Joint Strike Fighter is, at long last, ready to be unleashed in combat.

The announcement comes after years of testing and development, marking a significant milestone for the sometimes-beleaguered, often-criticized and always controversial $400 billion program, which is years behind its original schedule and billions of dollars over its original budget.

The declaration means that the squadron of 10 F-35Bs stationed with Marine Fighter Attack Squadron 121 in Yuma, Ariz., are “ready for worldwide deployment,” the Marine Corps said in a statement. And in announcing the decision, Marine Corps Commandant Joseph Dunford said the stealthy fifth-generation fighter “will transform the way we fight and win.”

Officials at the Pentagon and Lockheed Martin, the Bethesda-based manufacturer of the plane, cheered the announcement, saying it was evidence that the program had turned the corner from its previous troubles, and that it was primped and primed for its operational debut.

Air Force Lt. Gen. Christopher Bogdan, the program’s executive officer, said that the teams behind the plane “have worked through a number of challenges.” But he said they stayed focused on “delivering a stealth fighter that could fly faster than the speed of sound, carry its weapons internally, conduct short takeoffs and vertical landings, and be deployed from amphibious ships and austere bases.”

The fighter, the most expensive weapons system ever procured by the Pentagon, still faces questions from its legion of critics, however. Members of Congress recently asked Dunford, who is also President Obama’s nominee for chairman of the Joint Chiefs of Staff, pointed questions about the program.

“Do you believe the nation can afford to procure these aircraft at a cost of $12 [billion] to $15 [billion] per year for nearly the next 20 years for an aircraft design that will be 30 years old at the completion of the program procurement phase?” they asked.

They also pointed out that the aircraft is still under development and that full production is not scheduled until 2019, 17 years after the program’s inception. And they wondered whether the Pentagon really need 2,443 of the planes “in light of countervailing pressure to reduce force structure to conserve resources.”

Dunford replied that the F-35 “is a vital component of our effort to ensure the Joint Force maintains dominance in the air.” And he said that there will be updates to the plane over its life that will ensure it “maintains a tactical advantage.”

But he also said that the Pentagon is “analyzing” whether the 2,443 planes it plans to buy “is the correct number.”

That set off waves of concern, because the more planes the Pentagon buys, the less expensive they are. The Pentagon had dropped the number from its initial goal of 2,852. Another decrease could not only lead to an increase in price but spook allies, such as Canada, which are weighing whether to buy the aircraft, analysts said.

“Like all these major weapons systems sometimes, you just want to see them walk in a straight line and not fall off the curb,” said Byron Callan, a director of Capital Alpha Partners. “Hitting this milestone, which they’ve been talking about for months, if not years, is a positive step. But is it going to fundamentally alter the perceptions of the program? I’d say no.”

In a statement Friday, Sen. John McCain (R-Ariz.), one of the program’s leading critics, said he remains “concerned about the capability and reliability this aircraft,” and vowed to make sure the program continues to improve.

The F-35 comes in three variants, for the Marine Corps, Navy and Air Force. The Air Force variant is expected to be declared ready for combat sometime late next year, the Navy’s in late 2018 or early 2019.

The F-35B, a Marine jet that is going through operational testing, can land vertically. (Marine Corps)

The F-35B, the Marine Corps’ version, is the most complicated. It has a giant fan in the middle of the plane that allows it to land vertically. Before declaring it combat-ready, the Corps recently showed off the plane’s abilities as a small fleet of the jets took off and landed from the deck of an amphibious assault ship.

While the F-35B has now reached what’s called its “initial operational capability,” the plane’s development is not complete. There are still updates to the software that need to be implemented. But Dunford said Friday that he had full confidence in the plane, and that it was capable of an array of missions, including “close air support, offensive and defensive counter-air, air interdiction, assault support escort and armed reconnaissance.”

For weeks, the Marines put the plane through a series of tests aboard carriers at sea, live ordnance sorties and “large-force” exercises designed to gauge how the plane would perform in combat.