In  (page 27) Engelking states that:
The Niemytzki plane was defined (and attributed to Niemytzki), by Alexandroff and Hopf in 
which is accurate since in , where the plane is defined there is a footnote attributing that example to Niemytzki.
Does anyone know whether there is an article or book by Niemyzki where he defines the plane?
 Alexandroff, P., Hopf, H., Topologie I, Berlin 1935.
 R. Engelking, General Topology, Heldermann Verlag, Berlin, Sigma Series in Pure Mathematics 6, 1989.