Hydrogen technologies show promise in reducing the effects of aviation on anthropogenic climate forcing. This report aims to develop the design of a robust, low-maintenance, low NOx emission hybrid hydrogen-electric powertrain for retrofitting the Beechcraft 1900D by 2035. An optimal sizing of the two powerplants: high-temperature proton-exchange membrane fuel cells and a gas turbine with a rich-burn, quick-mix, lean-burn combustor, was performed, minimizing both the cost and climate performance of the design. In this optimization, the storage system, electrical system and thermal management system were sized utilizing technology projections for 2035. Additionally, exhaust water from the fuel cell stack is injected into the combustion chamber to further reduce NOx emissions. The resulting design has a passenger capacity of 15 with a range of 707 km, at a ticket price of €221 and a climate impact of 29.29 kgCO2,eq per passenger per flight, which is four times more sustainable than the current Beechcraft 1900D at a similar price point. The aircraft produces 1.1 grams of NOx on a typical flight, and never emits more than 8 ppm at any stage. The fuel cell provides all of the aircraft power during cruise and other low power flight phases, whereas the combustion chamber provides additional power during takeoff and climb. It is recommended to monitor the development of hydrogen technologies so they may be implemented in this retrofit by 2035.