See-through/transparent aircraft
It is possible, but not using typical modern construction techniques.
You have to build a structural frame and then clad it with a non-structural skinning. This technique was widely used in the early days, although the skinning was doped fabric and not transparent.
The frame technique is still used for many ultralights and hang gliders, and some transparent films are strong enough to be used for the skinning. But these types seldom have a significant fuselage. One exception I can think of is Paul MacCready's man-powered Gossamer Condor and Gossamer Albatross prize-winning man-powered aircraft, which had a fuselage nacelle slung under the wing, containing pilot Bryan Allen and the drive train.
Some helicopters such as the Bell Sioux have such large &quot;fishbowl&quot; cockpit glazing that it exposes all the cockpit innards. The Edgley Optica was a prototype observation aircraft along similar lines.
Talking of cockpit glazing, some flying wings had a prone pilot with the glazing forming part of the wing leading edge. These included some of the Northrop and Horten designs.
The Junkers G.38 airliner accommodated some passengers in wing cabins and the glazing to these formed the wing leading edge.
So yes it can be done in principle. But materials such as plexiglass or mylar film are not really suited to such things as enclosing engines or resisting birdstrike, while strengthened glass or polycarbonate get heavy and expensive so that on most planes, only small windows are practicable.
Advanced materials such as structural glass are being increasingly used in architecture, and I have no doubt that the manufacturers and researchers will keep pushing the envelope to develop new materials and new applications. But I don't know of any research focused on airframes.