Japan has chosen the Lockheed Martin F-35 in the F-X competition, Japanese newspapers report, citing government sources. A decision will be formally made on Friday, Dec. 16.

Around 40 aircraft are required, says the Mainichi. Officials previously said the requirement is for 42. The selection may result in many more F-35 sales, however, since Japanese industry has urged that the aircraft chosen for F-X be built until the late 2020s.

Even at the low production rates typical of Japanese industry, that would result in deliveries of perhaps 120 fighters, partly replacing Japan’s Boeing F-15C/D Eagles — although the selection of the F-35 somewhat limits the industrial benefits to Japan.

If the media reports prove true — and it would be unusual for top Japanese newspapers to be falsely briefed on an imminent decision — then Japan’s choice also is likely to improve Lockheed’s chances of winning South Korea’s current fighter competition, called F-X Phase 3, for which 60 aircraft are required. South Korean procurement officials would be criticized for ordering aircraft of earlier design than those chosen by Japan.

In both north Asian competitions Lockheed’s rivals are the Eurofighter consortium, offering the Typhoon, and Boeing, which has promoted the F/A-18E/F Super Hornet to Japan and the F-15SE Silent Eagle to South Korea.

Reasons for Japan’s choice of the F-35 included its stealth and suitability for information networking, says another paper, the Yomiuri, adding that the Japanese government was satisfied with U.S. willingness to share information that Japanese manufacturers would need in working on the project.

The order for the first four aircraft, fully imported, will be in the budget for the year beginning April 1, 2012. Japanese companies will help make later units. Unless Japan relaxes its ban on arms exports, it is unlikely to be able to supply parts for F-35s built for other countries.

Lockheed’s industrial offer to Japan includes final assembly of the F-35 airframe. Building the necessary plant will cost Japan about $1 billion, industry sources say. Japan is not expected to fully manufacture the aircraft from basic materials, as it did with a succession of fighters from the F-86, local production of which began in 1956, to the Mitsubishi Heavy Industries F-2, the last of which was handed over in September.

Reuters reported that the $8 billion deal may help end six decades of isolation for the country’s defense contractors.

The hope for Lockheed is that assembling the F-35 in Japan will spur the pacifist nation to lift a ban on military equipment exports, allowing contractors such as Mitsubishi Heavy Industries to compete as suppliers for the fighter.

“If the government chooses to go forward and relax the [export ban], we believe there is a very strong case for participating in the F-35 program,” said Dave Scott, director of international business development for the F-35.

While each maker disagrees on the merits of their competing bids, all agree that Japan has technology they could use. And for U.S. military planners juggling smaller budgets, widening out into a more competitive supply chain may let it arm itself more cheaply.

Although Japan is the world’s sixth-biggest military spender, it often pays more than double other nations for the same equipment because local export-restricted manufacturers can only fill small orders at a high cost.

Removing the ban would stretch its defense purse further as military spending in neighboring China expands.