NATO issued the specification NBMR-4 in 1960 requesting a medium-range S/VTOL transport to service fighters and strike aircraft at rough, unprepared airstrips. Dornier was one of the many companies offering designs. The Do 31 was the only design offering VTOL operation in addition to STOL. It used two Bristol Pegasus turbofans and eight Rolls Royce lift jets. NATO dropped the specification by 1967.
Dornier continued with the project until 1970. The aircraft flew with the Pegasus first in February 1967. The third prototype flew with the lift jets and Pegasus combined in July 1967. It began vertical trials in November 1967. The company continued with the program until 1970 before cancellation. Without a buyer, Dornier could not continue. Issues with the aircraft appear related to poor payload and high cost. The excessive weight, bulk and drag of the lift jets was blamed for the performance issue.
Bristol Siddeley built the BS100 vectored thrust engine for the super/transonic Hawker P.1154 during this same time perid. This engine provided 27000 lbf thrust in normal operation. It also included a Plenum Chamber Burning system, allowing 33,800 lbf for a short time. The Pegusus offered 15,500 lbf. The eight lift jets produced 4,400 lbf each. Total lift with the Pegasus/lift jet combo was 65,700 lbf. The BS100 offered 67,600 lbf.
What if Dornier had opted to use the BS100 instead of the Pegasus, and drop the lift jets altogether? Would the Do 31 have become a viable V/STOL transport? Or would the costs have remained excessive? Although NATO dropped the requirement, several planes entered were developed into long serving transports. The Do 31 appears to have a rather short range, 1800 km with the maximum payload of 3500 kg, using the Pegasus plus lift jets. I wonder how much the payload and range could have been improved with development.