There is an example of the difference between wartime development of aviation and peacetime. In 1914 American and European aircraft were roughly comparable. In April 1917 apart from Curtis Flying boats that were being sold to the Entente their were no American aircraft fit for use at the front. Even 19 months later when the war ended there were no US designed aircraft able to survive frontline service. The only US aircraft I can think of that was equal to, or possibly better than its European counterparts was the Curtis JN4 trainer.
This is a very good point. Perhaps the things we should consider are that prior to the war the US was spending 1.5% of GDP on defense while European powers were spending much more, 3-4%. As the US GDP as double that of European Powers this works out about the same in overall spending. The European powers were in competition with each other and as a market, Europe dwarfed the continental US 4-5 times over. Perhaps, without the war the Europeans would be pulling ahead of the US anyway.
Another thing to consider is that the Military didn't have the fastest aircraft in the world. They were several years behind the bleeding-edge tech. The other thing that the Military needed was aircraft that had a lifecycle of about 5 years. During war, this compressed down to 1 or 2 years. Having said that, the Military strived for aircraft that were maintainable in the field and easy to fly for the vast numbers of pilots. Without the war there would be a smaller number of more highly trained pilots.
Another thing to consider would be as theories around airpower evolve, what would be banned by international agreement. The Hague convention was held every 7 years and due in 1915. There was already a "Declaration Prohibiting the Discharge of Projectiles and Explosives from Balloons".
Here is a graph showing the numbers of unique designs by year. It shows the rapid growth to 350 different types per year to rapidly drop off as the war started and the ship went to vast numbers of a few types.
Looking at the development of the RFC, at first progress was slow and by 1912 the Air Battalion only had eleven qualified pilots compared to 263 in the French Army Air Service. By May 1915, the Royal Flying Corps had 166 aircraft.
By the time the Battle of the Somme started in July 1916 the RFC had a total strength of twenty-seven squadrons (421 aircraft), with four kite-balloon squadrons and fourteen balloons. The squadrons were organised into four brigades, each of which worked with one of the British armies.
By the end of 1917 the British has established their superiority over the German airforce and by the end of the war the RAF (RFC & RNAS combined) operated 4,000 combat aircraft and employed 114,000 people. The really big increases didn't happen till 1917-18.
For the RNAS, in 1911 the first 4 pilots were trained. By June 1913 44 Officers and 105 other ranks were trained as pilots. In August 1914 the RNAS had 55 seaplanes 38 aircraft, 6 airships, 2 balloons, 727 personnel (217 pilots).
When the RNAS was merged with teh RFC to form the RAF, the RNAS had 55,000 officers and men, 2,949 aircraft, 103 airships and 126 coastal stations.