This paper focuses on the design and experimental test results of a belt-driven starter generator for 48-V mini-hybrid powertrains. The functionalities required of the system are engine startup, launch assistant torque, regenerative braking, battery charging, and early fuel cutoff. The cost-effective electrical machine must provide high starting torque and wide constant power speed range, both in motor and generator modes. The authors outline the motivations of the electrical machine topology selection in relation to the specifications and constraints. In particular, a double three-phase induction machine has been selected as a suitable candidate. The aim of this paper is to summarize the design challenges of such kind of electrical machines, mainly subordinated to electromechanical and thermal issues imposed by the considered hybridization solution. Details of the machine design are presented, including a study on multilayer bar stator windings and on rotor slot design. Finally, predicted and measured performances of the prototype are reported and discussed for validation purposes.