Fifth generation fighters, such as the F-22 Raptor and F-35 Lightning, have provided the US with a certain element of aerial superiority Even though Russia's PAK FA remains in testing and engine related concerns continue to plague the development of China's Chengdu J-20 aircraft, external observers still consider the F-35 to be the technically better aircraft despite fifth generation fighters still sitting in the wrapper and without resting on its laurels, the US Air Force and Navy are already looking forward to the future. The F-22 Raptor and F-35 Lightning have been revolutionary in terms of their development as fifth generation fighters, yet preliminary work has already commenced on the design and development of their successors.