Asia is an arms race hotbed right now. China isn't exactly keen to play along with its neighbors, especially in regards to a bunch of small islands dotting the Asian continental shelf (particularly the island of Formosa, which in America we call "Taiwan"). Even that aside, they share an extensive border with another country called "India" that they tend to rub the wrong way, too. The Asian defense industry (and by extension, ours - and yes, that pronoun applies to you European and Russian Jalops too) is booming, and the holy grail of tactical technology right now is of course stealth. Dozens of nations are trying to obtain this technology one way or another, most of them taking the most straightforward option of buying it. Singapore is a partner in the F-35 Joint Strike Fighter stealth program, and the People's Vietnam Air Force operated advanced Su-30s which incorporate various forms of passive and active (electronic countermeasure) stealth. The five big players in the Asian stealth game, however, are Japan, Korea, India, the People's Republic of China and the Republic of China (Taiwan). This article will focus on the nation closest to winning the stealth arms race right now, Japan.
Japanese Stealth Pre-History
The Japanese stopped making warplanes in 1945 due to this event called "losing WWII." About ten years later they started making armaments again due to this thing called "the Cold War." The first indigenously-assembled jet fighters in the Cold War period - licensed copies of the American F-86 Sabre - were delivered to units in the late 50s complimenting ex-USAF stock. At the same time production also started on the Fuji T-1, brought to you by the parent company of Subaru. The T-1 was a (for the period) high-performance training aircraft designed to mimic the characteristics of a typical fighter jet. As such, and despite no American design input, being entirely designed by the Japanese, the T-1 absolutely looks like an F-86 stretched for a second seat. It was powered by an engine that was also entirely designed and manufactured in Japan.
The T-1 was replaced by the Mitsubishi T-2, which again was designed to mimic the performance characteristics of jet fighters at the time (this time throwing in supersonic capability and radar). So much so, that it was itself turned into a fighter known as the F-1 and earned the distinction of being Japan's first indigenously-designed tactical combat aircraft since WWII. Actual air-to-air combat was very secondary on the F-1's list of priorities; it was primarily developed to counter enemy warships, which Japan saw as the major threat against its trade and commerce traffic (and the Mitsubishi-assembled F-4EJ and F-15J was available to handle air-to-air missions anyway). It also ended up looking a lot like another Western type, the British/French SEPECAT Jaguar. This time it might not have been a coincidence, as the Japanese contemplated joining the British and French instead of developing their own aircraft and technical information may have been traded.
The Vietnam-era T-2 and F-1 combo eventually needed replacement itself, especially since the F-1 was no longer seen as adequate for Japan's defense against sophisticated threats. The F-4EJ was also reaching obsolescence, and the F-15J fleet was far too small to pick up the slack. The requirement was thus drawn-up for a "multi-role" fighter that could handle sea, air, and ground threats including the advanced Soviet-block MiG-29 and Su-27 fighters along with improved defenses against very high-performance anti-aircraft systems such as S-300 mobile air defense complex. Thus, Japan's first interest in stealth technology was born.
The earliest "official" depiction of the original FS-X concept I could find is this image, originally taken from Secret Projects
The original concept for what was then dubbed "FS-X" (Fighter, Strike, eXperimental) was a very radical departure from anything else then in production, as seen in the image above. The lines suggest some possible derivation from the F-15 (which Mitsubishi had extensive experience with the design from assembling them) but the design's most prominent features were a "reverse" configuration with canards up front and a surprisingly small planform wing at the rear, with twin engines and twin, likely canted tails. The canted tail feature is particularly interesting as it's a "low-hanging fruit" approach towards decreasing an aircraft's radar cross section (RCS) as seen on the F/A-18 Hornet and F/A-18E/F Super Hornet. It's possible if not likely that the FS-X would've incorporated low/medium passive stealth characteristics commonly found throughout "4.5 Generation" tactical aircraft such as the aforementioned Super Hornet, Dassault Rafale and Eurofighter Typhoon.
As found on F16.net this is claimed to be a developmental study model from Mitsubishi, displaying further evolved stealth characteristics including redesigned air inlets.
This fantaciful CGI depiction from defense.pk postulates what a fully-evolved, production FS-X may have looked like, displaying typical "4.5 gen" stealth features such as canted tails and semi-faceted air inlets akin to those found on the F-117 and F-22 stealth fighters.
Ultimately, FS-X was re-organized and Mitsubishi contracted with Lockheed-Martin to develop a cheaper alternative, an evolved version of the F-16 which would feature a larger wing and a host of construction and design innovations which, compared to a standard F-16 are mostly invisible. The F-2 reached operational status in the middle of the last decade, with the two-seat F-2B replacing the T-2.
Commitment to Stealth Technology
The desire to abandon the original FS-X concepts for a cheaper alternative are understandable; even the realized F-2 program ran into cost overruns and became an expensive enterprise, receiving much criticism regarding government expenditure. Another possible reason why the FS-X program was reorganized was the realization that the F-22 would soon be operational with the USAF. The F-22 would be a far more effective stealth platform than the FS-X in any form, and perhaps the Japanese assumed that the Americans would make the F-22 available to them, particularly as part of a licence production agreement.
Unfortunately, Congress passed House Resolution 2266, otherwise known as the Obey Amendment (after its chief sponsor, Rep. David R. Obey) which expressly forbade the export of the Lockheed-Martin F-22 Raptor to any nation, including close allies - let alone the production of F-22s on foreign soil.
In response, the Japanese government commissioned the development of the Mitsubishi ATD-X, or Advanced Technology Demonstrator-eXperimental. As the name implies, it's merely a, well, technology demonstrator. The design itself is far too small for a viable combat aircraft (smaller than even the F-35 and with no room for a weapons bay) and the shape of the aircraft is not meant to reveal any definitive production planform behind it. Rather, it is meant to demonstrate the development of Japanese technologies that would then lead to a full "Gen 5" tactical stealth aircraft. You can think of it as being similar to how the Bell X-1 demonstrated supersonic flight - in other word it's meant to show that, Yes, Japan can build a stealth fighter.
Depending on who you ask, that's exactly the point - the project is meant to show the Americans how silly their little Obey Amendment is and in the end it protects exactly nothing. There are no secrets to hide, because the Japanese have the technology to copy those exact same secrets without even needing direct observation - so Congress might as well just hand over those F-22s and be done with it. Congress has yet to agree to that logic, and very certainly never will.
Meanwhile, the F-35 Lightning II Joint Strike Fighter exists as a probable option. As the title "Joint Strike Fighter" implies, the F-35 is designed to defeat air, land, and sea-borne threats, which is exactly what the Japanese are looking for. The F-35 also fully possesses "Gen 5" stealth technology, though not to the extent of the F-22. Best of all, it's fully designed to be export-friendly from the get-go, being designed not by a single nation but by an international consortium much like the Eurofighter Typhoon. Indeed, very late in 2011 the Japanese formally announced their intent to purchase F-35As to compliment their F-2 fleet (which had suffered attrition from tsunami damage), partially replace the F-15J and help replace whatever F-4EJs were remaining. Coming this late to the party means Japan will see F-35s pretty late, though Lockheed-Martin promises deliveries can be expedited now that Japan has been cleared for partial production of F-35 components. There are also concerns about the F-35's performance against the latest versions of the Su-27, namely the Su-30MKK (that last "K" is particularly significant - it stands for "Kommercheskiy" i.e. "Commercial Export" i.e. "A Lot Of Our Potential Enemies Will Be Using These Things") and Su-35. And if nothing else, there's something about wanting one thing and getting the runner-up. The Japanese wanted the F-22. Perhaps the Japanese still wanted the F-22. And the Japanese have already demonstrated that they can make their own F-22. The Japanese love making their own things because it means domestic jobs programs and fighting off the Brain Drain. And now that they will be (partially) making their own F-35s, it's almost a matter of asking, "why not?"
This concept image taken from The Dragon's Tales shows the 23DMU (2011 Digital Mock-Up) and 24(2012)DMU. The 23DMU appears to be an "ultimate evolution" of the ATD-X, while the 24DMU bears strong resemblance to the Northrop YF-23 (which was turned down in favor of the Lockheed Martin F-22) and Sukhio T-50 prototype.
Bill Sweetman of Aviation Week reports that interest in the development of an indigenous stealth fighter may still be strong. However, there's a little bit more into this story. Sweetman himself reports that these "digital mock-ups" (illustrated and captioned above) are, like the ATD-X, mostly demonstrative and theoretical and are apparently used to evaluate F-2 upgrades rather than an actual stealth design.
This image, also from The Dragon's Tales, reveals "23DMU" has having an extremely strong resemblance to the Chengdu J21 prototype stealth fighter. "24DMU" appears to have a nearly perfect side planform matching the Sukhio T-50, especially the radome and canopy. China, which hopes to field the J21 as one of its two premiere stealth fighters, is (aside from North Korea) the nation Japan most anticipates having armed confrontation against. Both the J21 and T-50 are also meant for possible export, which in turn raises the possibility of Chinese-aligned or unaligned "Red Force"/"OPFOR" nations obtaining these aircraft. The resemblance to these two specific aircraft supports the theory that "23DMU" and "24DMU" are "virtual aggressor aircraft" and not representative of Japanese stealth aircraft efforts...but you never know.
A fantaciful image on an "ultimate ATD-X evolution" which also bears strong resemblance to the "23DMU" concept. More images are available at the source, which is here.
The above video shows that, if nothing else, Japan has its share of warhawks who imagine easy victory over any aggressor (implied to be China due to the inclusion of Su-30s/J-11s and what's presumably the Chinese carrier Liaoning). It also features an enlarged, production derivative of the ATD-X (logically called "F-3" in the video), carrier-launched(!) F-35Cs and a tail-less delta-wing design that bares a very strong resemblance to the cancelled FB-22 strategic strike fighter, perhaps yet another "ultimate evolution" of the ATD-X optimized for heavy strike (a role the delta wing is well-suited for, hence the FB-22 being redesigned around it). This heavy strike fighter features almost comically large engine bulges, which not only imply stealthy "s-ducts" (hiding the engine fan blades, which when spinning at peak RPM have a RCS somewhere between that of Fenway Park and the Empire State Building) but also possibly particularly large weapons bays and/or fuel cells (especially as the rest of the aircraft appears flush on the bottom). The Aviationist has further discussion, as does this blog.