Lockheed Martin’s F-35 Lightning II Joint Strike Fighter will boost Japan air defenses in the face of growing threat from China and Russia. (Courtesy of Lockheed Martin Corp.)

Japan is set to select the Lockheed Martin F-35 Lightning II JSF as its new stealth fighter, Japanese news media reported Tuesday, upgrading its air defense at a time when China and Russia pose growing threats to its territory.

The multibillion-dollar deal with the U.S. defense company also opens new opportunities for Japan to cooperate with Washington, its closest ally and military partner.

Japan will purchase roughly 40 fighter jets, the Yomiuri newspaper said, and as part of the deal, it will receive classified information about the F-35’s construction, allowing some of the fighter’s components to be manufactured in Japan.

The Japanese government will make a formal announcement about the decision on Friday, both the Yomiuri and the Kyodo news agency said. Japanese government officials often provide off-the-record briefings to the domestic news media in advance of major decisions.

Since September, Japan has been weighing bids from three of the world’s largest defense contractors. In addition to Bethesda-based Lockheed Martin, Seattle-based Boeing offered the F/A-18 Super Hornet, and a four-country European consortium, Eurofighter GmbH, offered its Typhoon. The decision for Japan came after years of declining defense budgets — a stark contrast to the rapid spending increases of an increasingly aggressive China.

Defense analysts had pegged the pair of U.S. bidders as the favorites, with diplomacy influencing the selection. But Japan also picked the priciest of the bids — a fraught decision at a time when the country must uncover untold billions to finance the reconstruction of its disaster-devastated northeastern coastline. Among the bidders, the F-35 uses the most advanced technology — with so-called “fifth-generation” design and a top-of-the-line stealth capability. The deal could be between $6 billion and $8 billion, according to estimates from defense experts.

Amid several high-profile territorial disputes with China and Russia, Japan has grown sensitive to incursions from its neighbors, often scrambling planes to chase off Chinese and Russian fighters. Japan’s newest defense strategy emphasized the growing threat of China, which this year will increase its military spending by more than 12 percent. Earlier this year, China tested its own stealth fighter, the Chengdu J-20, which features its own fifth-generation stealth technology.

Tokyo will receive its first four F-35s in 2016, the Yomiuri said. Eventually, the planes will replace the fleet of antiquated F-4s.

The F-35 has faced criticism for years of delays and cost overruns, with per-plane costs nearly doubling during development. But the Pentagon still expects the plane to come into service in 2016, and the Air Force, Navy and Marines plan to purchase more than 2,000 planes over the next 25 years.

Japan intends to import the first four planes that it will acquire in 2016, Kyodo said, but in the following years, it expects a hand in some of the production. Such a move would help domestic defense manufacturers, who now struggle under a national ban that restricts weapons exports.

The weapons export ban is a fundamental part of Japan’s pacifist constitution, but politicians have recently indicated an interest in relaxing the law. That would give Japan a place in joint development projects — a move that would be welcomed in Washington.