The Dornier Do 31 is an experimental, jet-propelled, vertical take-off and landing (VTOL) cargo aircraft that was designed and produced by West German aircraft manufacturer Dornier.

The development of the Do 31 was motivated principally by heavy interest expressed by the German Air Force in the acquisition of short take-off and vertical landing aircraft (STOVL)-capable aircraft. Such ambitions received a further boost from the issuing of NATO specification NBMR-4, which called for a VTOL-capable tactical support aircraft that would be operated in conjunction with the EWR VJ 101, a West German VTOL strike aircraft designed under the NATO contract of BMR-3.] A total of three aircraft, two flight-capable and one static airframe, were constructed and used for testing. On 10 February 1967, the Do 31 performed its maiden flight; the first hovering flight of the type took place during July 1967.

In addition to performing test flights, Dornier often demonstrated the Do 31 prototypes to officials and the general public, such as at the 1969 Paris Air Show. Several world records were set by the type during its limited flying career. When the high cost, technical and logistical difficulties of operating such an aircraft were realized, the German Air Force opted to cease trials involving VTOL aircraft, such as the Do 31, VJ101, and the later VFW VAK 191B. In the face of limited sales prospects and a lack of state support, the Do 31 and other VTOL projects lingered as research projects for a time prior to their manufacturers abandoning all activity. The Do 31 remains the only VTOL-capable jet-powered transport aircraft to ever fly.