Very Interesting!
One important tip I could give you about character designing is putting you spirit into each character including the antagonist. Since artists seem to focus on the main protagonist too much they may tend to forget about the other characters. Put your time and effort into each and every character ranging from the main ones to the least minor ones. Their names could be based on real pilots. For example, my name, Erika Grislawski. "Erika" was inspired by a marching song and Grislawski was inspired by German Ace Alfred Grislawski. And remember to choose your style. For example, the cartoony versions where characters have big eyes or the more realistic versions which characters have characteristics closer to human anatomy. To get familiar with these styles, you only have to watch watch read and watch anime and manga regularly. It takes time. But it's worth it. For the "styles and characteristics" of the characters, you must ask yourself if you're doing a realistic or the other type of manga where someone has pink, purple, or orange hair regardless of race. If you're doing the realistic version, your characters might have blonde, brown, light orange, and some black considering they are German characters. You might also want to give them Green eyes, Blue eyes or brown eyes. Hairstyle, hair color, and eye color really effects a character's first impression in anime and manga. And for the other version, your main character may have orange hair if he's a reckless, hot headed but gentle person who fights for justice(ichigo? lol). A girl with pink hair also reflects innocence, modesty, and kindness. A girl with long brown hair and glasses might reflect neatness, shyness, and determination. Don't forget to work hard on the eyes also . And as for the plane designs, if you're doing an alternate history manga, (this is getting interesting) you might want to modify the Bf-109 by swapping the position of the wings into a delta wing arrangement along with giving the delta wings a "swept" design. You might also want to modify the German stealth fighter, Me-262 jet or even the Haunebu anti-gravity flying disk. Your story would be much more interesting if you add some references of Nazi German superscience and wunderwaffe(wonder weapons) like the proton gun, the laser gun, the guided missile, or even "DIE GLOCKE"(the bell) to include TIME TRAVEL in your manga. "The bell" Is a bell shaped device developed by Nazi Germany for unknown purposes. At the end of the war, the SS Colonel in charge of the project including the scientists under his command ordered the massacre of slave laborers working on the project. Later, the SS Colonel, the scientists and the GLOCKE were all gone without a trace. Modern scientists suspect that the GLOCKE was actually a time travelling device. Could the Nazis be plotting revenge in another period of time? Will they come back?.... I hope these ideas would help you with your manga

Maybe I can give you some ideas^^ But as you know I always see it from the german view, but you don´t have to use it, only if you want^^

I would chose a german pilot coming from a family of nobility, that means he has a "von" before his surname. For example (my favorite name **) Ludwig von Reichel, or Friedrich von Hohenlohe.

Hm the look. Depends on what you like. I´d make my main-character look a bit like me maybe, that would mean brown hair and light-blue eyes. As a pilot he should also look like a gentleman, like the red baron

You could invent fantasy-countries, that are based on the real countries, for example:

Germany: Germania
France: Gallia
England: Anglia

Tho that would be just the latin names XD Or you call Germany Großpreußisches Kaiserreich (Great-Prussian Emperor-Empire) or Heiliges germanisches Reich deutscher Nation.

And then you make the borders you like.

In the story, Russia (maybe call it Moskowita) could attack Germania, which is allied with the Alp-Empire (Austria) and during a flight-attack, his brother gets lost and is captured by the enemy-soldiers. Then the hero tries to fly to the enemies and to liberate him in a secret mission with his friends.
Maybe his brother could have important war-informations, so that the high-command gives him the mission to help him.

His friends should come from different regions of Germany, so that they can tell different things about their homes or make jokes, who is the best and such stuff^^

I can also help you with names and phrases.

The flag of Germania or whatever you call it, should be a prussian-blue flag with an Iron Cross on it, I´d love that^^ Or golden background.

And you should use planes, that are already more developed, like the plans of "Nurflügler":

[link]

And I´d say Germany and England are allied against France and Russia. England must fight mostly at the sea and with the RAF, while Germany makes an awful land-war against the fascist Russians and revanchist French, who lost in the 1. world war against Germany. Maybe Austria is also already united with Germany here.

So far my thoughts XD I would also support your project, if you make it from german view, because I have much knowledge about german history, symbols and so on.

You could have a hybrid Napoleonic version of World War II, for starters.