HUNTSVILLE, ALABAMA—China has already begun defining its sixth-generation combat jet as the People's Liberation Army Air Force introduces its first fifth-generation type, the Chengdu J-20. Helped along by aircraft technology stolen from the West, the J-20 and exportable Shenyang J-31 are the most sophisticated, low-observable aircraft ever pursued by Beijing, and are designed to counter the Lockheed Martin F-22 and F-35 – which is being purchased by neighbors Japan and South Korea.