This uncertainty held until the second, improved prototype first flew on December 26, 2016. The new J-31 prototype is three tons heavier and about 20 inches longer than the original technology demonstrator; it also had key improvements like an IRST sensor, stealthier wings, cleaner burning engines, and an improved radar. In addition to avionics and datalinks that enable sensor fusion, SAC officials state that the production J-31s (which could appear soon as 2019) could have supercruise capability, giving them a leg up over current F-35 fighters. Its WS-13 engines would be replaced by domestic WS-13E or WS-19 turbofan engines to give it that advantage in speed. The combination of the J-31's high speed performance, and suggested payload of 6 PL-12 or 4 PL-21 long range air to air missiles suggests that the J-31 has been optimized as an air superiority fighter, though it can be fitted with a wide array of Chinese precision guided munitions like the LS smart bombs.