State-of-the-art wind turbines blades are large and complex structures, consequently making design challenging. The complexity is largely a result of using laminated composites which are high-performing and versatile materials, but also inhomogeneous hence exhibiting anisotropic behavior as well as challenging failure mechanisms. Structural optimization has been shown to be an invaluable tool for solving such challenging design problems, however a framework for treating all relevant parameters, i.e., material choice, fiber orientation, stacking sequence, and layer thickness, on the current scale is not available. Thus, the motivation of the present work is to present an overall gradient-based approach, where the key structural criteria are considered for multiple design load cases. The optimization framework is based on a variation of the Discrete Material and Thickness Optimization approach, where the thickness is directly parametrized, allowing for appropriately treating the sandwich parts of the blade. Application of the developed framework leads to an optimized design consisting of complex variable-thickness laminates, a good overall distribution of the structural criteria in the model, and a 23% reduction in mass compared to the initial design.