What's a similar looking aircraft to this illustration?
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:


domed cockpit at front of hull
hull chines
four radial engines mounted in-wing with three-blade propellers
conventional empennage



Photo of a Short Empire, Public Domain


Photo of a Short Sunderland, CCASA3.0 Nick.D

The largest all-metal flying boat developed for civilian passenger service was the Saunders-Roe Princess which only had two passenger-decks.


Photo of a Saunders-Roe Princess, Public Domain

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.


Photo of Hughes H-4 Hercules, Public Domain