@Flashman:
I favour one round only for air and naval combat.
With planes, up the combat value of aircraft built on subsequent turns:
Starting aircraft* = 1
Built turn 1 = 2
Built turn 2 = 3
Built turn 3+ = 4
You can mark the planes with wing stripes.
Give each power an extra starting plane in its capital; extra F in Lorraine, extra G in Munich.
Number of surviving aircraft greater than the enemy after 1 round of combat = the number of artillery you may boost. No surviving enemy and you get to boost all artillery.
Aircraft get to attack ground targets at combat value only if there is no air combat.
This is almost precisely what I was thinking! My only suggestion would be to have 2 turn intervals for the damage increase, so starting planes and those buillt on turn 1 = 1
Those built turns 2-3 = 2
Built turns 4-5 = 3
Built turn 6+ = 4.
This would encourage players to buy planes early, rather than waiting just a turn or two and have their investment be worth triple.
Starlight sniper-
Im liking this idea more and more; you could even save on transports as US by landing them on ireland and flying them to Picardy the next turn to rendevous with a small force of infantry landed there. And if america landed a few infantry across europe, they could then create a highly mobile force which could fly about the continent shoring up weak points in the allied lines. A growing American fighter force flying between Lorraine, Belgium, Venice, Albania, perhaps even Russia or the MI would either gain instant Allied air superiority across the board (blunting massed Axis artillery attacks) or force the Axis to invest heavily in aircraft.
