A Riesenflugzeug (plural Riesenflugzeuge, German for "giant aircraft"), sometimes colloquially referred to in English as an R-plane, was any member of a class of large World War I German bombers, possessing at least three aircraft engines, although usually four or more engines. These large multi-engine aircraft could fly several hours with larger bomb loads than the smaller Grossflugzeug bombers such as the Gotha G.V.

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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 adding a lowercase "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^{[note 1]} with a span of 29.8 m (98 ft), the Caproni Ca.4 with a span of 29.9 m (98 ft), the one-off Felixstowe Fury flying boat with a span of 37.5 m (123 ft) and the Handley Page V/1500 with a span of 38.41 m (126.0 ft). The Riesenflugzeuge that bombed London during the First World War were larger than any of the German bombers in use during the Second World War.^{[note 2]} The largest built, the Siemens-Schuckert R.VIII of 1918, had a wingspan of 48.0 m (157.5 ft). It was not until sixteen years later that an aircraft with a larger wingspan, the Soviet Tupolev Maksim Gorky eight-engined monoplane, was built with a 63.0 m (206.7 ft) wingspan.

The Riesenflugzeuge were operational from 1915 to 1919 and most of them were built as "one-off" aircraft.