We introduce C-tubes, 3D tubular structures composed of developable surface strips. C-tubes can be understood as a generalization of Monge surfaces---a special class of sweep surfaces---towards the recently introduced cone-nets. This observation allows formulating a constructive algorithm to create tubular structures that ensures developability of the constituent surfaces, while significantly broadening the design space. Our novel form-finding tool enables design exploration by solving for the input variables of the constructive algorithm so that the C-tube best conforms to user-specified objectives. We discuss several case studies that illustrate the versatility of our approach for the design and fabrication of complex structures, with applications in architecture, furniture, and lighting design.