This dissertation introduces a framework to carry out the aeroelastic optimization around the steady-state for flexible fixed wings. Such a framework captures geometrical nonlinearities and aerodynamic unsteadiness, which are the challenges in modern aircraft design. This also takes full advantage of structural geometry by integrating multi-fidelity structures with vortex-lattice method (UVLM), allowing effective and accurate multi-stage aircraft design with a variety of aspect ratios. Functions of structural consistency are used to close the gap between multi-stage designs at different fidelity levels. A model update tool is used to ensure structural consistency and allow communications between stages and fidelity levels. In addition, this framework shows its potential to involve more disciplinaries, such as control and stability, by revealing sensitivities with respect to UVLM vorticities and grids.The lower-fidelity aeroelastic model integrates nonlinear strain-based beam elements with UVLM, while the higher-fidelity model integrates nonlinear displacement-based shell elements with UVLM. The resulting aeroelastic models are coupled with a gradient-based method, focusing on overall performance and detailed analysis, respectively. An artificial neural network (ANN) is established for the model update from the lower- to the higher-fidelity structure. It builds a database and performs a statistical approach to address the parameter inequality. Aerodynamic equations are linearized by performing small perturbations and assuming a frozen aerodynamic geometry. The developed UVLM and aeroelastic models are validated by reference models. Analytical aerodynamic sensitivities are verified by the results of finite-difference. In addition, the feasibility of the model update tool is demonstrated by generating a two-dimensional shell and a three-dimensional wing box from original one-dimensional beams. The feasibility of this framework is demonstrated by optimizing a fixed wing under a large upward bending deflection. The impact of different design variables is also observed and discussed. This research illustrates a comprehensive process for an early design of flexible wings. Although the current framework is limited to the incompressible flow and isotropic shell elements, it has the potential to involve more applications with compressibility corrections of UVLM and composite materials.