Wikipedia
A Riesenflugzeug (plural Riesenflugzeuge, German for "giant aircraft"), sometimes colloquially referred to in English as an R-plane, was a large World War I German bomber, possessing at least three aircraft engines, more usually being powered by four or more engines, sometimes of more than one make, model or power level. These were large multi-engine aircraft capable of flying several hours with larger bomb loads than the smaller Grossflugzeug bombers such as the Gotha G.V. Some of the earliest Riesenflugzeuge were given G-type designations before being redesignated, but a major distinction was that the requirements for the R-type specified that the engines had to be serviceable in flight. As a result, designs fell into two groups - those with the engines mounted centrally inside the fuselage using gearboxes and driveshafts to transfer the power to propellers mounted between the wings, and those with conventional powerplant installations mounted in large nacelles or the nose of the aircraft where engineers would be stationed for each group of engines. The transmission of power from the centrally mounted engines to the remote, most often wing-mounted propellers proved troublesome in practice and most operational examples of Riesenflugzeug-class aircraft were of the second type, as with the all-direct-drive Zeppelin-Staaken R.VI.
The Idflieg (Inspektion der Fliegertruppen (Inspection of the Air Force), the German Army department responsible for military aviation), assigned the letter R to this type of aircraft, which would then be followed by a period and a Roman numeral type number. Seaplanes were denoted by the addition of a lower case "s" after the "R" in the designation.
The Riesenflugzeuge were the largest aircraft of World War I. In comparison, the largest equivalent Allied aircraft were the Sikorsky Ilya Muromets with a span of 29.8 m (97 ft 9 in), the Caproni Ca.4 with a span of 29.9 m (98 ft 1 in), the one-off Felixstowe Fury with a span of 37.5m (123 ft) and the Handley Page V/1500 with a span of 38.41 m (126 ft 0 in), of which only three had been delivered by the time the war ended. The Riesenflugzeuge that bombed London during the First World War were larger than any of the German bombers used during the Second World War, and the largest built, the Siemens-Schuckert R.VIII of 1918 had a wingspan of — it was not until sixteen years later that a larger-wingspanned aircraft, the all-metal Soviet Tupolev Maksim Gorky eight-engined monoplane was built, with an unprecedented wingspan.
The Riesenflugzeuge, most of which were built as "one-off" aircraft, were operational from 1915 to 1919.