Comment by m4rtink

Both A-10 & SU-25 have an armored bathtub made form titanium to shield the cockpit (somewhat) from ground fire, aimed mainly against heavy machine gun and auto canon fire + splinters. Sure, those are ground attack aircraft, not fighter aircraft but IMHO good enough.

Also IIRC some WW2 fighter planes had at lest armored glass front panel, to provide some protection from tail gunners when attacking bombers.