The Japanese government wants to develop a new, radar-evading warplane by combining elements of the U.S. F-22 and F-35 stealth fighters.

And now U.S. authorities are signalling they’ll release to Japan the secret technology that would make the hybrid fighter possible.

Japan has been here before. In the 1980s and ‘90s Japan licensed aspects of Lockheed Martin’s iconic F-16 fighter design and produced the F-2, a Japanese F-16 variant with a bigger wing and better electronics.

But the F-2 proved to be outrageously expensive. A uniquely Japanese stealth fighter, which would replace the small F-2 force, likewise could prove prohibitively pricey.

“The United States has proposed disclosing some of the top-secret details of its state-of-the-art F-35 stealth fighter jet to Japan to encourage joint development of an aircraft that will succeed the Air Self-Defense Force’s F-2 fighter,” The Yomiuri Shimbun newspaper reported.

The ASDF also has some F-35s. The U.S. plan, which was proposed to the defense ministry, would open the door to a jointly developed successor jet based on the F-35 and other fighters, which would be one of the world’s leading fighter aircraft.

According to Japanese government sources, the United States has indicated a willingness to release confidential details about the software installed in the F-35 airframe to control parts including the engine and the missiles. If the F-35 software, currently held exclusively by the U.S. side, is diverted to the F-2 successor aircraft, the United States will disclose the source code to the Japanese side.

The Japanese hybrid plane, which Tokyo likely would designate the “F-3,” could combine the airframe of Lockheed’s F-22 stealth fighter with the F-35’s sensors and electronics, according to one Lockheed proposal.

Japan in the early 2000s inquired about acquiring F-22s as replacements for the ASDF’s F-15s, but U.S. law bars Lockheed from exporting the F-22. Hybridizing the F-22 could allow Lockheed to sidestep the export-ban.

But Japan requires very few new fighters. The ASDF already has committed to buying 141 F-35As and Bs in order to replace the air force’s F-4s and oldest F-15s. The stealth fighters will fly from land bases as well as from the Japanese navy’s assault ships.

One Japanese F-35 on April 9, 2019 crashed into the Pacific Ocean, apparently killing the pilot and triggering a sprawling effort to recover the plane’s wreckage and any secret technology it might contain.

Once all 141 Japanese F-35s are in service, the ASDF could possess a mixed fighter fleet that also includes 102 upgraded F-15Js and 82 F-2s. Tokyo is proposing to develop the hybrid F-3 to replace the F-2s, but the same stealth fighter in theory also could replace the F-15Js.

But developing a new stealth fighter in order to buy at most 184 copies could prove extremely expensive. The F-2 is a cautionary example.

The modifications that Japan’s Mitsubishi made to the F-16, plus the F-2’s limited production run of fewer than 100 copies over 20 years, made it impossible for Japan to achieve economies of scale. It’s been claimed that an F-2 costs four times as much as an F-16, without providing anywhere near a fourfold increase in capability.

The U.S. Air Force acquired 187 F-22s at a cost of $70 billion. Taking advantage of existing technology could help to save Tokyo’s own new fighter program some money. The Yomiuri Shimbun estimated the fighter program’s budget at $18 billion.

Richard Aboulafia, an analyst with the Teal Group based in Virginia, for his part estimated it would cost Japan at least $20 billion to develop and build its own small force of stealth fighters.

That sum “certainly isn’t in the [ASDF] budget,” Aboulafia said. Japan spends just one percent of its GDP on its military. That usually translates into an overall annual military budget of no more than $50 billion, which is less than a tenth what the United States spends.

David Axe serves as Defense Editor of the National Interest. He is the author of the graphic novels  War Fix, War Is Boring and Machete Squad.