To expound on what Sky is saying, I built a series of plane sets for Europe, Mediterranean and the South Pacific in such a way that you can play each of those scenarios from 1939 to 1946 in either single missions or in campaign mode.
This allows the player to experience the rapid development of aviation during the war years. In this brief period of time you can start combat with late period bi-planes and end with early period jets.
Plus, in addition to the historical aricraft, I've included a lot of 'what-if' aircraft that either didn't make it past testing phase or in some cases were figments of some designer's imagination, the 'paper planes'. Think of the original Lucasart's SWOTL on steroids.
I'm now well into building an Eastern front set of planes to add to the mix, probably 2-4 months until it's complete.
The frontend software developed by Mr. Jelly make all the switching in and out of planesets, as well as scenarios, a piece of cake. In fact, you can mix and match scenarios too. Meaning, you could select the European theater and a set of planes from the Pacific, for example.
In addition to all that, there is software that allows you to build you own planesets from the available inventory. The learning curve is not daunting and there are detailed instructions to guide you along the way.