Italian Air Force Lieutenant General Paolo Civalleri told Reuters at the air show that his country was satisfied with progress on the plane. “Everybody is comfortable; the only problem is the budget,” Civalleri said.

Lockheed officials declined to identify any of the other countries exploring possible F-35 purchases, which are handled on a government-to-government basis, but said they had been engaged in nonstop meetings at the air show.

Lawson said the cooperative nature of the F-35 program, in which eight countries are chipping in to fund development of the new plane, would be increasingly important in coming years as budgets in the United States and Europe come under increased pressure.

Lockheed remains in protracted negotiations with the Pentagon about a contract for 30 more production jets, talks that have been under way since early December 2011.

The two sides remain at odds over overhead costs, with U.S. military officials asking for thousands of pages of additional documentation, on top of the 6,000 pages in Lockheed’s initial proposal submitted in April 2011. U.S. officials submitted their first counter-offer in April 2012.

Lockheed officials declined comment on the state of the negotiations. Navy Vice Admiral David Venlet, who heads the F-35 program for the Pentagon, did not attend the air show.

The four jets delivered this week will fly to Eglin Air Force Base in Florida, where Air Force and Marine Corps officials are getting ready to start training pilots later this year.

“We will be in full swing by the end of the year,” said Marine Corps Colonel Arthur Tomassetti, vice commander of the 33rd Fighter Wing at the Air Education and Training Command.