The United States has the most powerful air force in the world. Actually, there are four air forces, one each for the Air Force, Army, Navy and Marine Corps. Several other government organizations have large numbers of aircraft, but you have to draw the line somewhere. The American air forces have dominated the skies, worldwide, for over 60 years. There is no air force on the planet that can challenge this domination. With all that in mind, the one possible threat is the future. Who might build an air force that could threaten American air superiority?
The most likely threat is China, which has a booming economy, and an aggressive attitude in military matters. While China currently has nearly as many combat aircraft as the United States has (3,500), these are largely older designs (many MiG-21 clones) and new aircraft with reliability and durability problems (compared to Western planes.) But China is making an effort to replace more of those older aircraft with more modern designs, and provide better pilot training.
The average age of U.S. warplanes is about twenty years (navy planes, which are about a third of the total, average 15 years, while air force planes are over 20 years.) The 2,900 older aircraft (AV-8B, F-18C, A-10, F-15, F-16) are to be replaced by 2,400 F-35s (in three models). These will cost the U.S. about $240 billion. Nine foreign customers are buying about 3,000 F-35s, making the aircraft the most widely used warplane in the first half of the 21st century. The 31 ton F-35 can carry 7 tons of weapons (including a 30mm autocannon.)
The F-35 is to enter service in three years, with production continuing until at least 2034. That would require production to average about 200 aircraft a year. Including the $40 billion development expense, each aircraft would cost about $100 million. New technology down the road, cost overruns, and the growing popularity of UAVs (which may well replace some of the F-35s) will probably increase per-aircraft cost to over $120 million.
The increasing capability and reliability of UAVs are the big threat to the F-35. A UAV version of the F-35 has been investigated, but it's the smaller, cheaper, longer endurance UAVs that are the real threat. Persistence (the ability to stay over the battle area constantly) has changed the way battles are planned and fought. Air force commanders believe that this persistence is at risk against a well armed enemy, and an abundance of high-performance anti-aircraft weapons. But UAV advocates believe this problem can be coped with, and that the ground forces will not give up the advantage persistence has provided. One way or another, unmanned aircraft will drive manned combat aircraft from the battlefield.