Citing the existence of advanced Russian weapons
such as the S-400 air defense missile system and the Su-57 (formerly
PAK-FA) fighter, Air Combat Command commander General Mike Holmes told Aviation Week & Space Technology
that while the F-22 and F-35 will continue to be improved, "Eventually
you will run into a limit in your ability to improve those platforms,
and so we need to have something else ready."

The
U.S. hasn't designed a clean-sheet fighter in twenty years, and
Penetrating Counter Air (PCA) will differ from the F-22 and F-35 in some
ways to deal with new strategic realities.