China has had a reputation for stealing technology from Russia since the Cold War. After the collapse of the Soviet Union in 1991, China became one of the chief buyers of Russian-built Su-27 fighters and Russia allowed the Shenyang Aircraft Corporation to open up a production line to build the fighter for both the PLA Air Force and Navy, which was known as J-11.
China subsequently acquired a license from Russia to upgrade the J-11 into the more advanced J-11B. However, China became a potential challenger in the export market for military aircraft when it began to sell the aircraft to other nations.
To avoid Chinese engineers back-engineering its technology and claiming it as their own, Russia has chosen China's regional competitor India to develop the T-50 fighter together. Though India does not have the technology to develop a fifth-generation fighter, it can contribute through financial support.
China is working on its own stealth fighters, the J-20 and J-31.