When airplanes first got windows, what material was used?
Polymethyl methacrylate (Plexiglas)

The material was developed in 1928 in several different laboratories by many chemists.
Historically, PMMA was an important improvement in the design of aircraft windows, making possible such iconic designs as the bombardier's transparent nose compartment in the Boeing B-17 Flying Fortress.

From early on Plexiglas found a market in aviation, as the alternatives were either flammable or too heavy:
History (albeit short) of aircraft windows

The largest impediment to having closed cabins was the material the windows were to be made of. Prior to [Plexiglas] becoming available in 1933, windows were either safety glass, which was heavy, or cellulose nitrate (i.e.: guncotton), which yellowed quickly and was extremely flammable.


(wikimedia.org) B-17 nose Plexiglas.