The program marked a major success last month when Japan chose it over the Boeing F/A-18 and the Eurofighter Typhoon as a replacement for 42 aircraft in its aging air force. It was the F-35's first victory in an open-bidding competition, though countries from Britain to Israel previously made commitments and others are expected to follow.
Manufacturer Lockheed Martin also is looking to bring F-35s to South Korea in a deal that could be Seoul's biggest single defense outlay ever — 60 top-of-the-line fighters worth more than $7 billion. A decision could come as soon as October.
In the U.S., however, the stealth jet has been called a boondoggle. John McCain, the ranking Republican on the U.S. Senate Armed Services Committee, has slammed the F-35 as a "scandal and a tragedy," a "train wreck" and "incredibly expensive."
With U.S. defense budget cuts looming and many critics of the program still unconvinced, foreign support is a make-or-break issue for the program, which has been described as too big to fail. It could become the cornerstone of global air strategy for the next few decades, or a trillion-dollar bust.
"The U.S. fighter jet industry has all of its eggs in this one basket," said Richard Bitzinger, a security expert at Singapore's Nanyang Technological University. So many countries have bought into the program, he said, there is now no realistic choice but to forge ahead with it.
"It would be almost impossible for the U.S. to cancel the F-35, since the repercussions would be global," he said.
The F-35 is the world's only "fifth generation" fighter jet, combining state-of-the art stealth technology with highly advanced avionics and maneuverability. The first F-35 flew in 2006, and 42 have been produced so far. China and Russia are working on rival — and some experts say superior — aircraft.
About 130,000 people in 47 states and Puerto Rico have jobs related to the project. The only states without F-35 work are Hawaii, North Dakota and Wyoming.
"Simply put, there is no alternative to the F-35 program. It must succeed," Secretary of the Air Force Michael Donley said in September.
The Pentagon envisions buying 2,443 F-35s for the U.S. Air Force, Marine Corps and Navy, but some members of Congress and Department of Defense officials are balking at the price tag, which has jumped from $233 billion to $385 billion. Some estimates suggest it could top out at $1 trillion over 50 years, making it the most expensive program in military history.
In frustration over cost overruns, Congress added a requirement that Lockheed Martin cover extra costs on future F-35 purchases to the defense bill it passed last month.
"The delays and cost increases that F-35 has suffered have put it under substantial political pressure in Washington, so a win like the Japan program is a major boost," said James Hardy, Asia Pacific specialist with IHS Jane's in London.
Success rides heavily on foreign investment because the more F-35s are produced, the cheaper each jet is to build and maintain.
Lockheed Martin, in conjunction with Northrop Grumman, Pratt & Whitney and BAE Systems, has been careful to bring in international partners. The fighter is being developed with support from Britain, Norway, Denmark, Holland, Italy, Turkey, Australia and Canada.
Among the leading international partners, the U.K. is planning to buy 138 F-35s, Italy 131 and Canada 65. Australia has ordered 14 and has plans to buy as many as 100 for 16 billion Australian dollars ($17 billion).
The Israeli government selected the F-35A as its air force's next generation aircraft in 2010 — making it the first country to receive the F-35 through the U.S. government's Foreign Military Sales process.
Singapore also has said it will buy F-35, although it hasn't set numbers yet, and there may be longer-term interest from Saudi Arabia, the United Arab Emirates, Brazil and India, said Hardy, of IHS Jane's.
The wide range of buyers is in contrast to Lockheed Martin's last stealth fighter, the now discontinued F-22 "Raptor." It was hailed as a wonder of technology but failed in large part because Congress deemed it too sensitive to sell even to Washington's closest allies.
Narushige Michishita, a professor at the National Graduate Institute for Policy Studies who has advised the Japanese government on defense issues, said he thinks the F-35 is Japan's best option.
"If this was about a Cold War-type competition, then the F-22 would have been better. But if this is a long-term peacetime competition, you need numbers and presence, and close coordination among allies," Michishita said.
But defense analyst Carlo Kopp of the private Air Power Australia think tank said he thinks it was a mistake for his country and others to buy in. He said the F-35 program should have been canceled years ago and that the policy of pushing forward with it at any cost only threatens to create a budgetary sinkhole that would weaken the defenses of the U.S. and its allies.
"It will never become a viable combat aircraft due to cumulative poor choices made early in the design, and later Band-Aid fixes," Kopp said.
Further cost increases could prompt foreign buyers to cut their orders, which would put even more pressure on Lockheed Martin. Other problems also continue to trouble its international partners:
• Concerns about whether Lockheed will be able to deliver on time prompted Australia to caution that it won't decide until later this year whether to buy any more than the 14 ordered so far.
• Structural glitches have emerged that compromise the F-35's ability to land on aircraft carriers. That's a big issue for Britain, where the plane is slated to replace its carrier-friendly Harrier jets by 2020. British media have also reported that the F-35 can't fire British air-to-air missiles.
• Canada and Norway may have difficulty operating the F-35 on icy runways. The plane's single-engine design — unlike the twin-engine F-22 or F-15 — could also be an issue. If the engine goes out, planes and pilots in the Arctic could be lost.
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