 Five pre-series aircraft followed 1929, the first four differing from the prototypes in having a detachable ring mounting for a Lewis gun in the rear cockpit.The fifth, designated the R-Xa, was built as a strengthened, unarmed, long-distance tourer with a modified exhaust system and a fuel capacity of that gave it an endurance of 18 hours.The R-X was a braced parasol wing design of mixed construction (steel and wood) and conventional in layout.