The incorporation of unmanned aircraft systems (UAS) in its operational apparatus is recognized by the Portuguese Air Force (PAF) as potentially beneficial, particularly in the domain of intelligence, surveillance, and reconnaissance. Over the past ten years, the PAF has conducted several research and development programs with the objective of operationalizing UAS technology, keeping in mind its integration with the operational apparatus responsible for maritime surveillance and search and rescue (SAR) missions. In this context, it is the present work's objective to design a class I unmanned aircraft (UA), intended for maritime surveillance, atmospheric pollution monitoring, and SAR missions. Its development encompasses conceptual, preliminary and detailed design, focusing on the integration of systems, performance, aerodynamics, stability, propulsion and composite structural design. Mission requirements are set to comply with the European Maritime Safety Agency's specifications. The approach to conceptual design follows a methodology in which the airplane is conceived in a step-by-step, iterative fashion, making use of spreadsheets, empirical data and numerical information obtained with the software XFLR5. It starts with the aircraft concept generation and selection, as well as its initial sizing. Subsequently, the main wing, fuselage, and tail are designed. Finally, static and dynamic stabilities are evaluated, flight performance is assessed, and the flight envelope is determined, yielding the design load, which acts as a design driver, setting the aerodynamic and inertial load characterization of the critical flight maneuver. In preliminary design, the software Star-CCM+, a computational fluid dynamics tool, is employed in a main wing's geometry parametric study, performed with the aim of maximizing endurance. Other aircraft components are analysed, the design is refined, and the necessary onboard systems are integrated, resulting in the definition of the aircraft’s interior layout. The aircraft detailed design is performed through Computed Aided Design (CAD) modelling, with the objective of minimizing the structural weight. It is endorsed by a numerical stress analysis via a Finite Element Analysis (FEA), achieving convergence for both stress and displacement results. The developed design withstands the critical load condition without failure of the structure. As the project’s output, an aircraft capable of not only meeting but also surpassing the demanded mission requirements, is presented. The main wing parametric study concludes that the aerodynamic benefits fail to balance the structural and manufacture drawbacks.