Transitional aircraft are systems that combine the advantages of fixed-wing airplanes (e.g., high speed, range and endurance) and rotorcraft (e.g., vertical take-off and landing, hovering) by flying in fixed-wing and rotorcraft flight modes and perform transition between these modes. Research in manned transitional aircraft started seventy years ago before being abandoned at the end of the last century due to many problems in design, performance, control, and stability. In the last decade, due to the advancement in the Unmanned Aerial Vehicle (UAV), transitional UAVs regained great interest in the aeronautical society. Motivated by their significant capabilities, and given a specific set of design requirements, this paper aims to present a complete methodology (including all necessary mathematical formulations) to develop a transitional UAV comprising the following crucial aspects (conceptual, preliminary, and detail design, aerodynamics, performance, and stability analysis). The required battery capacity and total mass are determined from mission analysis that include both VTOL and FW mission segments. The design is iteratively resized when the actual components of the propulsion system are selected.