12 is a high end military craft ( much like the one in the comics ). same goes for 2, and 10 and 11 are pretty advanced too.
The vast majority of planes would fit in a pre-WW2 tech level, but some military planes are post WW2 level, tech-wise. ( 12 for instance, can almost reach mach 1, and a couple others can actually go past the sound speed ).
The tech in Oleon doesn't evolve like our own, as they don't need to rediscover everything, engineers know there was much more efficient stuff created before, and they can get their hands on books and stuff that can help them.
The problem in Oleon is not really the knowledge, but the material that is not easy to acquire. For exemple, petrol is very rare, you don't find that on flying islands, and offshore refineries are very difficult to build for various reasons. To sum up on that particular subject, most planes and airships don't even use kerosene or petrol-based fuel to run, and that's a limit in some cases. EVen if a state was able to build lots of planes like 12, they would not be able to run them. Engines are weaker, so planes need to be lighter, etc. You'll notice the heavier planes there have larger engines ( altho I love the design, 8 isn't right because of the engine that is too small ).
