Wood, once the material of choice for wind turbine blades, was phased out in the late 20th century as the growing size of blades imposed stricter material requirements and glass- and carbon-fiber composites gained industry popularity. However, the last several years have seen great advances in bio-based composite materials technology, including flax, hemp, and wood composites and laminates. These materials are increasingly utilized in high-performance, structurally demanding applications, largely because they are a more sustainable choice than many other engineering materials. Today, as the first glass-fiber wind turbine blades are ready to retire, wind developers are presented with an enormous challenge in disposing of these difficult-to-recycle blades. Through bio-based materials, the potential exists for these composite structures to be carbon neutral, renewable, and recyclable. In comparison to glass and carbon composites, bio-based composites offer numerous advantages. In addition to the environmental benefits, these materials have excellent specific strength and stiffness properties, meaning that they are very strong and very stiff but also lightweight. This has made plant-based fibers especially attractive for use in large wind turbine blades because of how critical a blade's mass is for turbine design. However, there are several unique challenges to the commercial use of bio-based composites compared to glass and carbon composites. These challenges include: limited availability of experimental data; a limited understanding of how bio-based materials behave under complex loading conditions; and the lack of a standard framework for computational modeling of these materials. This dissertation expands the current bodies of knowledge on wood laminates, flax composites, and wind turbine blade design by addressing these potential limitations. First, the treatment of shear properties of laminated wood is addressed by comparing several existing methods for determining shear strength and stiffness and proposing a new method based on tension and compression test data of multiaxial laminates. Second, a yield criteria analysis explains how wood laminates under multiaxial stress may be integrated into commercial finite element software for structural design. Third, a similar methodology is used to make failure criteria recommendations for multiaxial flax-fiber laminates. Finally, these results are used in combination with an aero-structural optimization routine to produce examples of large bio-based and hybrid wind turbine blade designs. The techniques developed herein have broad implications for the design of bio-based composite structures worldwide.