The Bitter CD, (based on the Opel Diplomat) a three-door hatchback coupe featuring a Chevrolet 327 V-8 of 227 hp (169 kW), was built between 1973-1979.

The CD was first shown as an Opel styling study OPEL CD (Coupé diplomat) on Sept 9th, 1969 at the International motor show (IAA) in Frankfurt. Designed by, Charles M. "Chuck" Jordan (OPEL Design boss 1967-1971 and later vice-president of GM) and his assistants George A. Gallion, David Holls, Herbert Killmer and Hideo Kodama, along with Erhard Fast (director/conductor of the OPEL Designstudios 3 for Advanced Design since 1964) was involved. The tail opinion is inspired by a draft of Erhard Fast for the OPEL Aero GT of 1969.

Due to large success of the IAA CD study. Opel played with the thought of building a road-suited vehicle to develop further. The doors should be replaced by normal doors, and the vehicle bumper, windshield wipers and a reuse of other Opel Diplomat parts for cost and ease of going to production. Robert "Bob" Lutz, who was head of Opel at the time, was inspired and wanted the car in production. So he commissioned Pietro Frua to work on the revision of the Opel CD and preparation of two driveable prototypes.
Rare Bitter SC sedan and convertible

In 1971, David R. "Dave" Holls (Opel design boss since 7/1971; previously assistant to Charles M. "Chuck" Jordan) encouraged Erich Bitter to build the Bitter CD. So Erich Bitter formed Bitter GMBH 1971, specifically for the CD. He started with a 1-acre (4,000 m2) site in his home town of Schwelm. Since he didn't have the capital or time to set up his own production facilities, and looked for a proven independent, small scale company to build the CD. He chose Baur of Stuttgart as they produced prototypes and limited production runs for several well known German companies. They possessed the necessary experience, had the capacity to build the car, and their build quality was impressive.

Erich Bitter drew some designs based on the Frua CD before he decided on later style elements. The basic changes of the Frua CD vis-à-vis Opel CD of 1969 in the Bitter CD design, were to cut off stern, modify the windshield and reduce the amount of chrome. Dave Holls and Opels team of designers, supplemented the design with a small front spoiler, added a larger grill, set the bumpers higher and extended the lower edge of the rear side windows to the rear window. Final testing and trials were done at the Opel Test Facility in Duden yards, along with simulated duration load tests initiated by Erich Bitter that were carried out on the Hydropulser in Baur in Stuttgart. The team at Baur performed also in addition considerable constructive and production technical product development inclusive the building of a hard foam model to mock up the body shell.

Baur's role in building the CD included making the body panels, assembling the shell, trimming and fitting the interior, and mating the Opel Diplomat running gear to the shell.

The Bitter CD was displayed, very successfully, at the 73' Frankfurt Motor Show, where Erich Bitter took 176 orders for his stylish new car. Unfortunately, the oil crisis soon after hit his plans and most orders were cancelled. Even though many of the orders were cancelled production commenced in late 1973 at Baur GMBH in Stuttgart. While production never reached the target of 200 cars per year, by the beginning of 1975 Bitter had built 100 CDs. By this time the oil crisis had receded and CD sales continued through to 1979.

1979 production ended with a total production of 395, (1973: 6 - 1974: 99 - 1975: 79 - 1976: 73 - 1977: 71 - 1978: 30 - 1979: 37), as well as 5 raw car bodies for the warehouse. Purchase price in 1974 was 58,400 DM.