This study documents the design, development, and evaluation of a purpose-built aerial firefighting fleet optimized for diverse wildfire suppression environments as part of the COLOSSUS project’s X-Challenge. The multidisciplinary effort encompassed aerodynamic design, propulsion system, systems integration, cost estimation, simulation, design of experiments, and fleet optimization. Key technical advancements include a conceptual hybrid electric Vertical Takeoff and Landing (eVTOL) aircraft design, and the integration of a series hybrid propulsion model into the System of Systems Inverse Design (SoSID) simulation toolkit, in which evaluation takes place at fleet level. Simulation results indicate that the proposed aircraft achieves competitive or superior effectiveness across all test scenarios, with the series hybrid configuration offering notable endurance and tactical adaptability.