American intelligence believes that the tech behind the new Chinese J-20 stealth aircraft came from Russia. So far, not a lot of protests from Russia. This might indicate that MiG sold some tech to the Chinese, and didn't issue a press release. Of the four stealth fighters than have been built and flown so far (U.S. F-22 and F-35, Russian T-50 and I.42), it was noted that the J-20 looks most like the I.42, a MiG project that was cancelled in 1997. The rear portion of the J-20 is particularly similar to the MiG I.42. MiG has been having financial problems for over a decade, and was eventually bailed out by the Russian government, and absorbed into the new Russian Aircraft Corporation. Before that, it would have been tempting for a MiG executive to pass plans for the cancelled I.42 to the Chinese in exchange for a large amount of cash deposited in a Swiss bank account, or to simply do a legit deal, with government approval.
The J-20 made its first flight on January 11th, two weeks after it was seen doing taxi tests at a factory airstrip in central China. There are two J-20 prototypes, one with Russian AL-31 engines, and one with Chinese WS-10As (a copy of the AL-31). China is releasing very little information, and apparently accepted the fact that flight tests could not be hidden. Based on recent warplane development projects (J-11 in particular), the J-20 has a long development road ahead of it, and will likely change size and shape before it reaches the production design. Thus its quite possible that the J-20 is just a development prototype. The Chinese know how complex aircraft like the F-22 and F-35 are, and the large number of technologies required to create a true 5th Generation aircraft. China has already demonstrated its willingness to spend decades perfecting the capability to design and build advanced weapons systems.
While the shape of the J-20 confers a degree of stealthiness (invisibility to radar), even more electronic invisibility comes from special materials covering the aircraft. It's not known how far along the Chinese are in creating, or stealing, these materials. Same with engines. The current engines being used are sufficient for early flight tests, but not capable of providing the "super-cruise" (high speed cruising while consuming far less fuel than usual). Super cruise would be essential for the J-20, since China would most likely use the aircraft singly, or in small groups, to seek out and attack American carriers. Two years ago, China announced it was developing the WS-15 engine, a more powerful beast well suited for the J-20. No date was given as to when the WS-15 would be available for use.
For the J-20 to be a superior fighter, it would need electronics (including radars and defense systems) on a par with the F-35 and F-22. So far, the Chinese have not caught up with stuff used by current American fighters. But the gap is being closed, faster than it was during the Cold War and the Russians were creating, or stealing, their way to military tech equivalence with the West. Work on the J-20 began in the late 1990s, and the Chinese knew that it could be 25 years or more before they had a (hopefully competitive) stealth fighter-bomber.
The twin engine J-20 appears to be a 36-40 ton aircraft that is longer (at 24 meters/75 feet) than the F-35 or F-22. The F-35A is a 31 ton, single engine fighter while the F-22 is slightly larger at 38 tons. The I.42 was a 42 ton aircraft that was 19 meters long while the 37 ton T-50 19.8 meters long. The J-20 appears to be a fighter-bomber, as this kind of aircraft would be most useful dealing with the U.S. Navy and key targets in Taiwan or Japan. The J-20 appears to have more internal weapons space than the F-22, thus capable of carrying more weapons, particularly anti-ship missiles. It appears that China is planning on having the J-20 ready for service by the end of the decade. The key factor is their ability to develop or steal the needed technology by then.
Russian fifth generation fighter developments were halted when the Soviet Union disintegrated in 1991. Actually, all development work on new fighters, by everyone, slowed down in the 1990s. But work on the F-22, F-35, Eurofighter and Rafale continued, and those aircraft became, in roughly that order, the most advanced fighter aircraft available today. MiG resumed work on the I.42 in the 1990s, but had to stop after a few years because of a lack of money. Sukhoi has never stopped working on its T-50, funded by much higher sales of its Su-27/30 fighters. This fifth generation may come to be called the "last generation," after they are replaced by the second generation of pilotless combat aircraft (counting armed Predators and the like as the first).