Carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) are promising candidates for the development of light-weight, nanoreinforced, multifunctional composites. Although BNNTs and CNTs share similar mechanical properties they can impart a different set of functionalities. While CNTs are electrically conductive BNNTs show transparency in the visible region, are electrically insulating, thermally more resistant than CNT and have the ability to shield neutron radiation. High nanotube (NT) contents are often necessary in order to achieve mechanical, thermal or electrical conductivity requirements for a particular application. However, it is still challenging to attain a simple and economical method that improves nanotubes dispersion and interfacial interaction with the matrix. We have developed a process for the fabrication of high-loading polyurethane (TPU)/NT nanocomposite sheets using a one-step solution method that is easy to scale-up. Through optimization of the processing conditions it was possible to control the nanotube dispersion and the morphology of the nanocomposite sheets. The fabrication of 30 x 30 cm2 NT/TPU composite sheets was demonstrated showing the scalability of the method. A comparative study of the influence of BNNTs and CNTs on the tensile properties of the developed NT/TPU sheets was undertaken. After incorporation of pristine nanotubes, superior tensile properties were obtained with CNTs (modulus, strength and failure strain of 1 GPa, 40 MPa and 100%, respectively) suggesting a stronger interaction of CNTs with the TPU than pristine BNNTs. After functionalization of the BNNTs a significant improvement was observed with 100% improvement in stiffness compared to CNTs. In the case of CNTs, electrical conductivities up to 7000 S/m have been obtained. The nanocomposite sheets are flexible, robust and can be integrated, similarly to other fiber fabrics, into conventional composite manufacturing processes to fabricate hierarchical structures.