The Yugoslav Royal Air Force turned to the Rogožarski factory for seaplane pilot trainers in 1937. A requirement was the seaplane would match the characteristics of the SIM-X. Since it was not possible to adapt existing SIM-X for installation of EDO floats, chief designed Sima Milutinović fitted the new, larger plane with a more powerful Walter Major 6 190 hp engine. At the end of 1937 the prototype was ready for testing. The first flight was conducted on February 7, 1938, by test pilot Ivan Koroša in at Divulje. After the flight tests had achieved satisfactory results, the first batch of aircraft was ordered, which were delivered in mid-1939.
The SIM-XII-H two-seat plane, with engine Walter Major 6 (190 hp), was of predominantly wooden construction, with an elliptical cross-section of fuselage entirely made of wood and covered with plywood. The wings were wood covered with fabric, with rounded ends. On each side, the wings are supported by a pair of inclined struts attached to the fuselage. The fuel tank was located in the central section between the wings. The first series of aircraft had two EDO Model 47 floats installed.
Nine were built in total.
The Royal Maritime Navy signed a contract with the Rogožarski factory on July 15, 1940 for a second series of these plane (4 aircraft) with these seaplanes equipped for instrument, or, "blind" flying. Rogožarski delivered these aircraft in five months but without floats as the delivery from Canada was delayed for several months. With agreement reached between the Rogozarski factory an the Navy command, the design and development of domestic aircraft floats was launched, unfortunately this project did not reach completion due to the outbreak of April war . In the pre-war period two SIM-XII-H aircraft were in accidents both in Boka Kotorska in 1940. Both aircraft were designated for disposal, so the Navy Command requested approval to install the floats from these planes onto new aircraft (onto the 2nd series of SIM-XII-H) considering that these planes were equipped with instruments for instrumental ("blind") flying. When the approval was given, the floats were installed into new planes, so the aircraft in the second series in use before the war broke out.
The type was retired in 1941.
Engine: 1 × Walter Major 6, 6-cylinder line, 140 kW (190 hp)