Looks like decals for the windows both cockpit and the passenger cabin, Rafael.
Just want to say that this is a super project, and gets better with each post. This machine simply looks like something out of those late 30's Popular Mech/ Popular Science magazines. Its got Art Deco, futurism and basic workable engineering going so well together.
Suggestion: you will be fitting props soon enough and there are two ways to treat that, either a small bullet hub -exposed pitch rings, basic cover or you could consider a aerodynamic spinner, a larger cover. I think the latter would add to the look you have created and are approaching. Also, a similar deco stripe like on the tails should go on that front landing gear spat, plus the other two.