the first airplane to have a lavatory

The word is from middle English, in turn from middle French: lava - to wash and tory from toire - room, thus "washroom", in practice of course meaning a toilet. According to one source, the first plane to have one was the Ilya Muromets (Sikorsky S-22), manufactured in Riga, Latvia, in 1913.

The Lawson L-4, a 1921 trimotor biplane manufactured by the Lawson Air Line Company for use in its night flights between Chicago and New York, had sleeping berths, a lavatory and shower (below).

Outside of aircraft, the word lavatory is often used in the UK, but rarely in other English speaking countries.