Among all WWI bomber designs, German DFW biplanes earn a special mention, thanks to their unusual powertrain.
The first was the R.I (don't forget that "R" index is for Riesenflugzeug, i.e. giant airplane). Developed as a private venture by DFW, it was a large biplane of conventional configuration with four 220hp Mercedes D.IV 8-cyl. water-cooled in-line engines mounted inside the fuselage, powering propellers on the wings via transmission shafts - two mounted tractor-fashion on the leading edge of the upper wing, and two mounted pusher-fashion on the trailing edge of the lower wing.
After factory tests proved promising, military acceptance trials commenced on 19 October 1916 and led to the aircraft being purchased for the Luftstreitkräfte. Soon thereafter trouble set in, with crankshafts repeatedly failing. This was attributed mostly to the engine design, but new engine mountings and universal joints for each end of the drive shafts were fitted to mitigate the problem, along with extended wings and other improvements.
The R.I in its final form; the machine in the foreground is a SSW D.1 fighter
Following these modifications, R.I (R 11/15) was deployed on the Eastern front with Rfa 500 at Alt-Auz, April 1917 to September 1917, from whence it raided Riga during the summer of 1917. On its second combat mission, the R.I crashed due to the failure of two engines, and was destroyed.
The DFW R.II was developed at a request by the Luftstreitkräfte in spring 1917 after their experience with the R.I had been generally positive.
The service desired generally similar aircraft but needed greater payload (3400 kg, up from 2600 kg in the R.I) and wingspan (35.5m). This meant the design had to be considerably revised. The same engine/propeller arrangement was used (four inline engines mounted in the fuselage, driving two tractor propellers and two pusher propellers via long driveshafts).
When the R.II first flew in August 1918, the driveshafts proved troublesome, creating excessive vibration. As a remedy, they were enclosed within steel tubes, which fixed the problem. The aircraft also was able to benefit from the newly-available Mercedes D.IVa 260hp engine that had replaced the troublesome D.IV in production.
Of the six ordered by the Luftstreitkräfte, only two were completed before the end of the war, and these were operated from Cologne on training duties only when their performance proved inadequate for front-line duties. Following the war, DFW planned an airliner version of the R.II(above), which would have carried 24 passengers. Construction of a prototype was abandoned before it was complete.
Back to the bombers. Conceptually similar to DFW's preceding R.I and R.II designs, the R.III was to have been a much larger aircraft, powered by eight engines. As with the previous designs, these were to be housed inside the fuselage, driving propellers by long driveshafts. In the R.III design, however, these propellers were to be mounted on a nacelle in the interplane gap. Each end of this nacelle would carry two co-axial propellers, each driven by a separate engine. Spacious bay was planned for 2,500 kg bomb load. Defensive armament consisted of 8 machine guns. Had it been built, the R.III would have been the largest aircraft in the world at the time, its wingspan (53.5 m) superceding even this of the Mannesmann-Poll ten-engine triplane*.
Source: The German Giants, The Story of the R-planes 1914-1919, G.W. Haddow & Peter M. Grosz, Putnam & Company Limited, 42 Great Russell Street, London, First Published July 1962
* If you think the DFW's are not weird enough, wait for the Mannesmann-Poll - a superbomber designed for transatlantic raids!