It's a good question because there is no simple 'go to' listing.
This is a starter for ten.
https://www.bundesarchiv.de/EN/Navig...sed-fonds.html
Please be aware that it hasn't been updated to include any of the holdings which have recently been digitised.
Get yourself registered.
I have found that one of the best ways to start off down this particular rabbit hole is to throw a relatively generic term, i.e. Flugzeug into the search engine and tick the digital files only box 'Nur Treffer mit Digitalisaten anzeigen' and then follow through. I strongly recommend spending some time playing with it to get your head around its very particular hierarchical way of cataloguing and delivering results but there are some good things to be found and downloaded FOC.