The US – which is developing its own futuristic replacement for the stealth bomber – has been the only global superpower since the end of the Cold War.
But is increasingly facing competition from China as well as Russia.
China warned war was "inevitable" after US warships sailed close to disputed islands in the South China Sea.
The Communist country is also developing a "scattergun" nuke capable of firing 20 nuclear warheads on the US.
North Korea is also preparing to test a new nuclear weapon – sparking fears of World War 3.