Redesign and modification in the development of F-35 Joint Strike Fighter (JSF) will continue to drive the cost of the program for the next few years, but it will likely start dropping in 2015, Vice Adm. David Venlet, the Pentagon’s program executive officer for the JSF, said recently.

Venlet said the redesign and modification costs–known as “concurrency”–have been affecting all aspects of the program, including hardware, software, schedules and technical performance, but he was confident that Pentagon and manufacturer Lockheed Martin [LMT] were making progress in tackling the problem.

“Concurrency is very real in the program at this time…,” he said. “It will persist probably to the beginning of 2015 and then it will recede rapidly.” By 2015, the Pentagon should have a good handle through fatigue testing to make any additional changes, Venlet said at a forum hosted by Credit Suisse and Jim McAleese and Associates on Thursday.

Concurrency costs have been a key reason behind the budget overruns on the program, and, along with technical problems that prompted delays, have contributed to the Pentagon decision announced last month to remove construction of 179 of the aircraft from the five-year defense plan.

The Pentagon said it was slowing down procurement to allow more time for testing and to make developmental changes to minimize concurrency costs before moving into large-scale production.

The Pentagon plans to buy 2,443 F-35s in three variants for the Air Force, Navy and Marine Corps for a $282 billion price tag–the largest acquisition program in the building’s history.

Lowering the number of F-35 buys over the next five years has prompted concerns it could increase the cost of each warplane, including from some other countries committed to buying the F-35. Venlet said he has assured Japan, which announced plans in December to buy 42 JSFs, the budget change won’t impact its deal.

“We have given them assurances that out 2013 adjustment did not make their deal invalid,” Venlet said.

Venlet said the partner nations in the program–the United Kingdom Italy, Netherlands, Turkey, Canada, Australia, Denmark, Norway–are keeping a close eye on whether the Pentagon can bring down concurrency, but he was confident they will remain in the program following a March 2 meeting at the Canadian embassy in Washington.

“I have no indication that anybody is thinking about leaving the program,” he said.

The Pentagon and Lockheed Martin are negotiating the procurement of the fifth batch of the planes in low-rate production. Despite fewer buys than planned, Venlet said he was confident the two sides can reach a “fair and reasonable” agreement to keep the planes affordable.