Slitherine

I have a Japanese B17 and Stuart. Yet when they take losses I am unable to reinforce them at all.

Are they supposed to be like this? A one pony trick to show off and then sell the next mission or something? I wouldn't mind paying a lot of resources to reinforce my B17 to use against the american bases.

It's pretty logical.
IRL Japan can't build B-17 or Stuart.

And they would not have spare parts etc.
When the unit has a very low strength you can upgrade it to a Japanese unit of same type, ie tank.

Yeah the idea is that you can either use it as a free, unique unit until it runs out, or you can use it to "train a crew" and then upgrade it to a standard Japanese unit once it gets worn out.