I generally* try and play a historically plausible game, so I plan to slowly iterate my models over time, with gradual improvements, as occurred during the period. Beaufighter Mk X here we come

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* I'll have the odd crazy game/achievement run from time-to-time as well

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I really am surprised that a lot of the posts above seem to be behind a system that gives best in game bang for buck to a concept of "it's best to save all your variant exp until you research a new model"
You should have to earn the experience in new model before you can improve it! After all the experience was earned on the old model.
Just an appeal, we need the game to have some semblance to reasonableness rather than maximizing gameness.
I'm fully in agreement with this - I'm not sure how I'd do it, but maybe have a maximum number of variant 'slots' that can be filled in before a new model has been built and seen combat. That way, you'd be encouraged to do the multiple versions and the like that actually occurred during the conflict. That said, I could understand if the devs think this might be a bit over the top in terms of detail (but deffo not too much for me personally). Fingers crossed it'll be moddable/changed down the track.