There is this crazy picture of a plane or a boat or both...?
This is just a drawing, I know, but if this aircraft does not exist, is there perhaps a similar looking one in reality?
is there perhaps a similar looking one in reality?
Not really, however the fictional five-deck flying boat depicted is clearly an extrapolation of flying boats of the first half of the 20th century.
Visually, the Short Empire class is the most similar:
The largest all-metal flying boat developed for civilian passenger service was the Saunders-Roe Princess which only had two passenger-decks.
The largest flying boat was probably the Hughes H-4 Hercules, popularly known as the Spruce Goose. It was designed for military transport (hence no windows). I believe it would have accomodated both troops and military vehicles but probably no more than two decks.
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