In 1970, Xian Aircraft Factory proposed an upgraded four-engined version of their H-6 Tu-16 copy. The proposal was approved, while the PLAAF issued an extra requirement of capabilities against sea targets two years into development. To test their plans, Xian re-engined an H-6 with four Rolls-Royce Spey turbofans - two in the wing roots and two under the wings .Designated the H-6I, the prototype took flight in 1978. Performance gains were impressive - climb rate increased by 40%, and range increased by a third. Payload was also increased significantly. While Xian’s proposed H-8 was to use revised Spey engines mounted in underwing pylons to reduce maintenance costs, the program fell victim to downsizing efforts in 1980.