Dornier Do 31
This odd West German experimental plane was one of the largest VTOL aircraft ever flown. First introduced in 1967, the Do-31 could haul more weight than even the modern MV-22 Osprey. The system wasn’t a tilt-rotor aircraft though. Instead, it required four engines to operate Two Bristol Pegasus vectored-thrust turbofans in the inboard nacelles provided power in forward flight, and four Rolls-Royce RB162 on the outer nacelles generated lift for takeoff. While incredibly reliable, the aircraft was far from cost-efficient and never progressed beyond the prototype stage.