The U.S. government is pressing F-35 international partners to make a firmer commitment to the $391 billion program by signing on to a “block buy” of some 450 fighters over three years, Lockheed Martin said Monday at the Paris Air Show. That would lend certainty to a program that has thus far delivered only a handful of fighters to its eight partners.

“Each of the countries associated with the program are being asked right now if they’d like to participate in that, and this would be to make a commitment for the aircraft purchase,” said Lorraine Martin, Lockheed Martin’s F-35 program executive vice president and general manager. “A block buy enables us…to lock down three years of the program,” she added.

According to Martin a multinational commitment could comprise 460 to 500 U.S. and partner nation orders from 2018 to 2020–somewhat more fighters than earlier indicated. In a separate briefing on Monday, Mark Buongiorno, vice president of Pratt & Whitney’s F135 engine program, said the engine supplier is providing information to the U.S. Department of Defense (DOD) for an estimated buy of 150 fighters per year over that period.

“Right now, we’re supporting the DOD with studies to estimate what type of cost reduction will come from this higher production rate procurement,” Buongiorno said. “We’re looking at a three-year procurement [in the] roughly 150-range of aircraft, and engines and spares to go along with that.”

Frank Kendall, DOD undersecretary of defense for acquisition, technology and logistics, announced the “block buy” approach last month while attending the F-35 CEO Conference in Oslo, Norway. This is an annual meeting of partner nations and industry participants to discuss the status of the F-35 program. Lockheed Martin expects the concept will “play into [the] decision making” of partner nations as they go through their various procurement processes, Martin said.

Thus far, Lockheed Martin has delivered two F-35s to the UK, although they are presently based at Edwards Air Force Base, California and the Marine Corps Air Station in Beaufort, South Carolina; two to Australia, currently based at Luke AFB in Arizona; and one to the Netherlands, also based at Edwards AFB. The company expects to deliver a third F-35 for the UK later this year.

If all partner nations keep to their original procurement plans, the manufacturer expects that by 2020 half of production at its Forth Worth, Texas, facility will be for international F-35s.

Following Martin’s briefing, Lockheed Martin offered an indication of the pressure that is being applied as the F-35 program emerges from its system development and demonstration phase, which is expected to be completed in 2017. The manufacturer and Terma, of Denmark, signed a memorandum of understanding that would secure Terma as a supplier for the F-35 over the life of the program–but only if Denmark selects the F-35 for its fighter replacement program.

Lockheed Martin is offering Danish industry $5.5 billion in industrial participation over a production run of some 3,000 F-35s, said executive vice president Orlando Carvalho. If the Danish government does not select the F-35 “we would respect that decision,” he said. But the long-term participation of Danish industry on the program “would be something that we would have to revisit.”