Ammonia has emerged as a promising carbon-free energy carrier for sustainable aviation due to its high hydrogen density and well-established production, transportation, and storage infrastructure. However, the adoption of ammonia as an aviation fuel introduces significant challenges related to propulsion efficiency, fuel storage integration, and airframe structural design. This study proposes a multidisciplinary conceptual design and analysis framework for ammonia-fueled composite aircraft, integrating aerodynamics, structural mechanics, and fuel system considerations within a unified design environment. At the conceptual design stage, candidate aircraft configurations are generated and evaluated through coupled aerodynamic analyses and propulsion performance models representative of ammonia-based power systems. The composite wing planform is designed and optimized based on the wing area required to satisfy mission range constraints. A multi-objective Bayesian optimization framework, combined with two-way coupled aeroelastic analysis and structural sizing, is employed to simultaneously minimize aerodynamic drag and structural weight, thereby enhancing overall aerodynamic efficiency. For each optimized wing configuration, the composite fuselage is subsequently sized to ensure structural safety and integrity. The influence of wing planform parameters, particularly wingspan, on total airframe structural weight is systematically investigated. In parallel, a conventional kerosene-fueled aircraft is analyzed as a reference case to quantify the impacts of ammonia fuel storage requirements on airframe weight, load distribution, and overall aircraft efficiency. Key performance metrics, including lift-to-drag ratio, structural weight, and mission feasibility, are evaluated to identify viable design trends and multidisciplinary trade-offs. The results provide valuable insights into the conceptual design of next-generation ammonia-fueled composite aircraft and offer guidance for future sustainable aviation development.