TRAVERSE CITY, Mich.  General Motors said Thursday that it had “essentially finished” designing its first plug-in hybrid car, the Chevrolet Volt, and would have production-ready prototypes within 10 days.

The automaker still has considerable work to do on the car’s lithium-ion battery and other technology in the two years before the Volt is scheduled to go on sale, but completing the design is a milestone for what is arguably the most crucial car in decades for G.M.

The Volt would be able to travel at least 40 miles on battery power alone, G.M. said. The battery is recharged by plugging a cord into a household outlet.

Bob Boniface, the director of design for the Volt, showed sketches of the car and photos of its front and rear corners at an industry conference in northern Michigan. He said G.M. had made the Volt more aerodynamic and attractive since displaying it as a concept car at the Detroit auto show in January 2007.