Electric vertical takeoff and landing (eVTOL) aircraft have gained widespread attention in urban air mobility services recently. In the design of eVTOL aircraft, the powertrain system significantly impacts its performance and cost. This paper systematically describes a conceptual design and optimization methodology for eVTOL aircraft from the powertrain perspective, emphasizing its influence on overall aircraft performance and cost. The powertrain modeling methodology is achieved by considering a set of converter topologies and cutting-edge component technology, including wide-bandgap semiconductor devices, high-power-density passive components, and thermal management. Apart from that, a hybrid optimization algorithm is used in this paper to facilitate a rapid global search. Los Angeles has been selected as the example city for this study. A case study with a 30 km cruise mission range is conducted, analyzing the variation of each design variable within the optimization process. Moreover, a comparative study is implemented to illustrate the effects of the powertrain system on the eVTOL aircraft. Finally, a sensitivity analysis of the cruise mission range is presented. This paper addresses the gap in current eVTOL design methodologies from the powertrain perspective, further reducing the mission cost of eVTOL aircraft within the revenue mission profile.