Almost certainly (The National Interest: "Indeed, in some cases, highly classified data on some of the United States’ most advanced military hardware—including the Lockheed Martin F-22 Raptor, F-35 Joint Strike Fighters and Bell-Boeing V-22 Osprey—is in now in the hands of foreign adversaries as result of cyber-espionage.") (Original DNI report: "Adversaries will continue to use cyber operations to undermine US military and commercial advantage by
hacking into US defense industry and commercial enterprises in pursuit of scientific, technical, and business information. Examples include theft of data on the F-35 Joint Strike Fighter, the F-22 Raptor fighter jet, and the MV-22 Osprey.") Though they appear to be rather rough copies of 5th generation US aircraft.
China began the project called the J-XX, apparently in the late 90s parallel to our own development of 5th generation aircraft, which became infamous for cost overruns and innumerable delays.
J-XX 4th-Generation Fighter Aircraft"The Chinese aviation industry began the preliminary research for China’s 4th generation fighter programme in the mid-1990s. In 1998 the U.S. Office of Naval Intelligence (ONI) reported that an advanced F-22-class twin-engine stealth fighter known as J-12 was under development at SAC. Later in 2003 an Internet source photo revealed a fighter mockup for wind tunnel test which may be linked to the J-12 project. As more details of the fighter began to emerge, it was understood that the fighter will also be fitted with an internal weapon bay and possibly active phased array radar."
Due to the secrecy of the J-XX its hard to know how successful the PRC was during this period, and I would suspect that the project was probably more bark than bite in this period, despite what US Intelligence and the PRC would like us to believe (One must remember that the F-22 program cost 67.3 billion (150 million unit cost), and the F-35, depending on when the program is finished, will cost an estimated 428.4-1,196.4 Billion (115 million unit cost), during this period China was still suffering the after effects of the famine years of Mao and was spending vast amounts to industrialize, lacked a domestic military jet industry (relying instead on Russian MIGs), and when many Chinese lacked electricity due to a dire shortage of power plants).
"Considering China’s records in combat aircraft development, a project like the J-12 may prove challenging. It will involves technology advancement in a number of fields including materials, high-performance aviation engine, electronics, flight control software, and stealth technologies. A project of this scale will also require huge amount of investment and considerable knowledge of complex project and manufacturing management. While China may be able to benefit from some “off-the-shelf” dual-use technologies available in the commercial market, it will almost definitely seek assistance from its traditional military technology suppliers such as Russia and Israel. However, none of these two countries possess the experience of developing an advance fighter of this class."
The project most likely became much more substantial in 2012-2013, as a new Chinese President, Xi Jinping, ascended to power (note I included the dates 2012-2013 because he became General Secretary in 2012 but did not actually become the President until 2013). President Xi introduced a radical shift in Chinese foreign policy which continues to this day but of which the general American public did not truly begin to come to terms with until very recently. President Xi's aggressive change in both foreign and domestic policy is a strange blend of post-Maoist communism (authoritarianism combined with state capitalism), old school Nationalism, and Neoconfucian principles (Xi Jinping Thought).
Both China and Russia have recognized since the 90s that in order to have any substantial influence on the world stage that it would need to quickly modernize its military from the traditional Soviet force, that had arguably changed little since its success in WW2 (the AK-47 for example had been created in 1947, only two years after the end of WW2). The Gulf War made this need desperately clear as advanced US air power completely devastated the Iraqis' Soviet-style army. This lead to increased efforts to design more modern MIGs by the Russian Federation and for China to begin investing in the J-XX project. Unfortunately for both of them these projects would occur at a snails pace as the Russia Federation began a period of complete economic chaos and a brutal war in Chechnya would quickly prove how ill-equipped the modern Russian army was. This would change with rise of Vladimir Putin who would double down on the brutality of Russian forces in Chechnya and bring the conflict to a very slow and very bloody end. He also would increase military funding across the board, especially when it came to the development of more modern MIG designs.
China on the other despite seemingly realizing the danger of its outdated military made little real effort to modernize the PLA and instead continued to pursue its famously insular foreign policy. This would remain essentially their policy until 2012 with the rise of Xi who quickly made modernization of the PLA a top priority as he recognized its vital role in his new foreign policy. Evidence of this was clearly displayed with the introduction of the Shenyang FC-31 in 2012, likely based on the Lockheed Martin F-22 Raptor . More recently the Chengdou J-20 was officially introduced in 2017 (Jane's 360), however in reality the project seems to have been developed in parallel with the FC-31, with the J-20 being China's attempt to copy the Lockheed Martin F-35 Lightning II
Stolen F-35 secrets showing up in China’s stealth fighter: "The Chinese cyber spying against the Lockheed Martin F-35 Lightning II took place in 2007 under what U.S. intelligence agencies codenamed Operation Byzantine Hades, a large-scale, multi-year cyber program that targeted governments and industry.
Defense officials said the stolen data was obtained by a Chinese military unit called a Technical Reconnaissance Bureau in the Chengdu province. The data was then passed to the state-run Aviation Industry Corp. of China (AVIC). An AVIC subsidiary, the Chengdu Aircraft Industry Group, used the stolen data in building the J-20, said defense and intelligence officials familiar with reports of the illicit tech transfer.".
"China’s Communist Party-affiliated Global Times reported Jan. 20 that China obtained key technologies from the F-35 and incorporated them into the J-20
The newspaper did not admit stealing the technology, but stated that China “completely obtained the six key technologies” from the F-35."
Though certainly they are based at least partially on US military aircraft, using designs stolen through Chinese espionage and reverse engineering, they almost certainly were not working from a complete design as the planes appears to be very much a hybrid of domestic Chinese technology Shenyang WS-10, licensed Russian parts (Saturn AL-31), as well as stolen US technology. Its therefore difficult to know how close these planes actually are to their US versions, after all an Iphone and a Chinese clone of an Iphone may at first glance appear to be very similar, but unless we can compare the parts internally its hard to know how close the two products actually are. We should also be very wary of any Chinese state media reports on Chinese military advances, while these aircraft certainly appear to be effective when we watch or read accounts provided by Chinese state media, so do Chinese laser weapons (video).
Image comparision
J-12
Phoenix News: The deputy commander of the Chinese Air Force first exposed: the domestic fourth-generation fighter is about to make its first flight: "This is a simulation diagram of aerodynamic heating calculation of a new fighter early exploration model disclosed by public media (Source: International Aviation Magazine sponsored by Yihang Group)"
Northrop YF-23
"According to the only two photos released, the next generation of Chinese fighters is closer to the YF-23 loser of the United States' four generations of competitive selection, but the duck wings are added."
Chengdu J-20
F35
