What led to American air superiority over the Axis in World War II

One of the main reasons was from the planes themselves. The Germans had three fighters at the outbreak of war; the Me-109, the Focke-Wolfe-190, and Me-110. The Japanese had one good fighter that was considered invincible (all the others were unreliable), the Zero. And they stuck with those throughout the war. The Allies, on the other hand, had numerous fighters of varying abilities, giving the Axis pilots a constant mix of different planes up against them they have to learn about the hard way. Another factor that affected the air superiority was the (untouchable) manpower and manufacturing resources of the U.S. By the end of the war, a small percentage of the remaining German pilots were fully trained and experienced, and up against them were thousands of better trained, better equipped, and more experienced Allied pilots. By the end of the war the Luftwaffe was almost non-existent.

All factors have been mentionned by different OPs, but I will present you a summary of how the Axis aviation went from victory to defeat:

Step one: get tactical defeats

The battle of Britain, the battle of Midway, are some examples of tactical defeats of the Axis aviation. They were lost against an opponent that was not threatened by an other mean (no ground or naval offensive possible), thus the fact that the Germans/Japanese lost more aircraft was a criteria of tactical and strategic defeat. However, the losses were huge but not decisive for the timeframe of the war.

Step two: get points of attrition

The year 1942 was a year of attrition fight of all fronts, and this year leads to a turning point because of these battles. For the aviation, the fight on Malta, on Guadalcanal or Stalingrad were points of attrition were heavy losses on both sides allowed the capacity of production and training of the Allied states (and mainly USA and USSR) to express themselves. In 1941, there was also some of these battles of attrition like in North and East Africa between the Italian and the British air forces.

Step three: loose the battle of bombings

During all these battles, the Axis air forces did not inflicted major destructions to strategic objectives because they lacked heavy bombers (as mentionned by another answer). The British and Americans did not. And they stroke hevaily the Italian and German industries, leading them to fail in replacing many parts of their forces. Add the specific lack of oil and oil with high octane degree for Axis forces in both Europe and Asia theaters, and the air forces are particulary leveled down by these bombings. Note that Japan suffered mostly of the submarine war, because bombings on its territory started later.

Step four: Don't have a major technological upper-hand

Despite major advances in rocket motors, neither actor of WW2 had a real uper hand in technology. Some advantages were even in the Allied side with the building of four motor-bombers and the disposal of very good classic engines (like the British Merlin). On other factors, as mentionned by other answers, the Americans and the British had medium armed fighters that were well adapted to their opponents, while the Axis hat to developed specific twin engined fighters to fight the heavy bombers.

Step five: be also beaten on other fields

The Axis might have work around these events with great naval and land victories. But it did not obtain these victories, partly because of its unhability to gain the air superiority, partly because on land and sea, the Allies were also good challengers.

I feel it was really a critical mass issue, which was lost around the start of 1943, mostly in the east. Looking back at losses, air forces don't stand up so well when stretched for resources. Germany had the massive eastern front to deal with as well as constant harassment from the British.

-Too many experienced pilots were lost, which reduced the effectiveness of each plane, putting extra pressure on the inadequate production and development of planes.

Early in the war, the Germans had many advantages. The Bf 109 was one of the best fighters of the war. Particularly at the beginning it outclassed everything but the Spitfire. Fighting in the Spanish civil war gave the Luftwaffe experience, then a constant string of victories made them almost unstoppable. The fall started with the Battle of Britain. Hitler only reluctantly decided to attack after Churchill refused his peace offering. It was a massive loss of air power for nothing in return. It delayed and hindered Barbarossa, Hitler's real goal.

Then came the Soviets - who had a lot of aircraft, but initially almost all of them were lost on the ground. Germans had the air virtually to themselves, but a combination of massive manpower, production and development of great planes slowly ground down the Germans. A lot of air power was lost around Stalingrad, particularly ground support aircraft, allowing soviet artillery to get a leg up. By the time of the Soviet counter-offensive numerical superiority was lost, partly due to the movement of aircraft to defend North Africa from the allied landings. If there was a tipping point it was around this time, mostly on the eastern front. Hitler had thrown everything at the Soviets and lost. The Soviets were producing more planes than Germany even in 1941, and this disparity nearly doubled by the next year. What's more, these planes were equal and sometimes better than the German planes. The US and Britain were also drawing critical resources away from the east and giving direct support of lend-lease materials and resources to the soviets.

By the time the allies were pushing through Europe, the US had collected many victories in the pacific, the arena of carriers and planes. By D-Day, Germany had only 600 fighters left, essentially nothing. The US had massively increased production of planes and had developed some of the best single engine fighters of the war. German resources were far too overstretched to really have had any chance of standing up to the allies at this stage, even with the 262.

During 1943 it was inaccurate to claim air superiority on behalf of the Allies. The American Eighth Air Force strategic bombers were being (on occasion literally) decimated during the daylight strategic raids over Germany. Other than the short range Spitfire the Bf-109 and FW-190 were the superior planes and capable of operation AT 20,000’ and higher. American P-38s, P-39s, P-40s and P-51As were not in the same league. Only when the high altitude Packard Merlin Powered P-51 B was introduced at the beginning of 1944 did the tide change. The P-51Bs had the range and altitude performance to escort the bombers. The escorts were permitted to leave the bombers to engage German fighters as per a directive from Jimmy Doolittle. The Luftwaffe weakness was not planes but pilots (and fuel). The experience of the average German pilot was marginal when pressed into combat. By D-day the Allies established air superiority in Eastern Europe.

The Soviets also greatly improved their planes and pilot skill. However, the American planes that were inappropriate in Western Europe were much better suited for conditions in the Soviet Union in that most combat there was at low altitudes.
Still the Luftwaffe developed the best interceptor of the war with the ME-262 jet. But it was too little too late.

Once the Zero’s weakness was identified it became less of a threat. While it had great maneuverability at medium to low speeds, it lost its aileron command at high speed. When confronted with high speed vertical tactics it as much less of a factor.

The Allies, especially the US after converting it's huge auto industry to aircraft production, were able to produce huge numbers of aircraft, and trained crew to operate those aircraft, while Germany was not. Germany did make astounding numbers of aircraft considering the state of their industry, but they were not able to supply those aircraft with trained pilots.

Nor were the Germans able to improve their designs as rapidly. US/UK manufacturing and project management methods allowed changes to be incorporated reliably and rapidly, while changes to German production took far longer to implement. The B17 went from the C model to the vastly improved G model, and the P51 went from the early B to the penultimate D model, in less than two years. Both were produced in vast numbers, in their improved form.

Not being harassed by a bombing campaign was a major factor there - German factories were under constant attack, while US factories were unmolested, and by 1943, UK factories were relatively free of air attack.

The Germans held the edge in high tech with their jet fighters, but were never able to produce them (or more correctly, never able to produce the engines) in such numbers to make a major difference. The lack of rare metals to produce reliable gas turbines was also a factor - the average life of a Jumo 004 was in the 20-25 hour range.

The US oil campaign proved to be very successful - towards the end of the war, lack of fuel became a major problem for both Luftwaffe and motorized ground forces as well. Much of the strategy of the Germans Ardennes offensive was based on capturing Allied fuel supplies... which did not factor in how easily a fuel dump could be set on fire.

To a degree, Allied training methods were better. Their philosophy was to send their most experienced pilots home to train new pilots, while Germany (like Japan) kept their aces in combat until they died. This accounts for the very high number of 'kills' of the best German aces, 200 to 300 kills, while the best Allied pilots rarely got more than 20 or 30. They went home to pass their expertise along to a great number of new pilots.

The result of that policy was lower overall kills for the individual pilots, but a higher overall level of experience on the part of new pilots, so they were more effective and suffered fewer losses. By early 1945, most new German pilots didn't last more than one or two missions, while most new Allied pilots survived the war.

Some factors not already mentioned:

• American factories and assembly lines worked hard, and turned out huge numbers of planes. A notable one is Willow Run in Michigan, which produced the B-24 Liberator. The Allied GDP outpaced that of the Axis.
• The American and British air forces alternated attacks against Germany. Americans would go bombing during the daytime, while the British would do so at night. It was called "round the clock bombing"; the intent was that "the devil will get no rest."
• When enough American aircraft had been committed to bombings, the German ability to produce diminished.
• Eventually, American and British strikes happened deeper and deeper in Germany, hitting infrastructure. Alongside landings at Normandy, bombing of German targets further intensified.

Air coverage, if not superiority, was essential. But for backup by the RAF, the evacuation at Dunkirk would have been subject to the Luftwaffe.