The Checker Aerobus.
An excerpt from the 1964 Checker Motors Corporation sales brochure for the Aerobus gave the following description:
"The Checker Aerobus was the result of persistent requests from fleet owners to create a special purpose vehicle with the economy of a taxicab, the comfort of a limousine and the ruggedness of a bus. Built at the same plant, in Kalamazoo, Michigan, and using the same skills, craftsmen and materials as on the billion-mile Checker taxicab. The Aerobus was conceived as the first and only totally effective method of transporting 12 people short or extended distances and with complete comfort, outstanding safety and true economy. Checker built special purpose vehicles for over 60 years. Taxicabs that defied weather and time, fleet cars that provided the utmost in passenger satisfaction and operator efficiency. The Aerobus, however, took the taxicab and fleet economy, convenience and safety several steps beyond, and provides a totally satisfactory service for 12 passengers, and with room for luggage, packages or equipment and with style and power features never before offered in a commercial car. The Aerobus was built from the ground up for the purpose of moving people. Not a stretched car, not a converted stock car, but a built-for-the-purpose, built-to-last vehicle of amazing stamina and comfort."
More here & here.