Although it is the costliest weapons system in U.S. history and the single most expensive item in the 2013 Pentagon budget, it will face only a glancing blow from the sequester this year. And as the White House and Congress contemplate future budgets, those pushing for additional cuts may find it difficult to trim more than a fraction of the Pentagon’s proposed fleet, even though the program is years behind schedule and 70 percent over its initial price tag.

The reasons for the F-35’s relative immunity are a stark illustration of why it is so difficult to cut the country’s defense spending. Lockheed Martin has spread the work across 45 states — critics call it “political engineering” — which in turn has generated broad bipartisan support on Capitol Hill. Any reduction in the planned U.S. purchase risks antagonizing the eight other nations that have committed to buying the aircraft by increasing their per-plane costs. And senior military leaders warn that the stealthy, technologically sophisticated F-35 is essential to confront Iran, China and other potential adversaries that may employ advanced anti-aircraft defenses.

The biggest barrier to cutting the F-35 program, however, is rooted in the way in which it was developed: The fighter jet is being mass-produced and placed in the hands of military aviators such as Walsh, who are not test pilots, while the aircraft remains a work in progress. Millions more lines of software code have to be written, vital parts need to be redesigned, and the plane has yet to complete 80 percent of its required flight tests. By the time all that is finished — in 2017, by the Pentagon’s estimates — it will be too late to pull the plug. The military will own 365 of them.

By then, “we’re already pregnant,” said Air Force Lt. Gen. Christopher Bogdan, who oversees F-35 development for the Pentagon.

When the F-35 finishes testing, “there will be no yes-or-no, up-or-down decision point,” said Pierre Sprey, who was a chief architect of the Air Force’s F-16 Fighting Falcon. “That’s totally deliberate. It was all in the name of ensuring it couldn’t be canceled.”

The Pentagon has long permitted equipment to be produced while it is still being tested, with the intent of getting cutting-edge gear to warriors more quickly, but senior military officials said the F-35 takes the approach to new extremes. Doing so has served as more than a hedge against cuts — it has also driven up the overall price. The 65 aircraft that already have been built, and those that will be assembled over the next few years, will require substantial retrofits that could cost as much as $4 billion as problems are uncovered during testing, the officials said.