"No one wanted to build prototypes anymore," remembers Jurgen Barth, a Le Mans winner, Porsche's long-time head of customer racing and, crucially at that time, head of the governing body of world motorsport's sportscar commission. "The big manufacturers were looking to rallying and we wanted to give them something in which they could show their products in sportscar racing."
Manufacturers could develop a car out of a Group 2 touring car, which called for a minimum of 1000 units, or a Group 4 GT car, which required 400 examples. Porsche chose the latter route, of course, and got a head start with its 911 Carrera RSR in 1973. The car famously won the classic Targa Florio road race entered as a prototype as it tried out developments, including turbocharging, that would be incorporated into the 935.
There were different ways to skin a cat, however. Porsche went for more and more wild versions of the 935 as Singer, not to mention many of the marque specialists such as Kremer Racing from Cologne, came up with more and wilder interpretations of the rule book. BMW, a participant initially with both normally-aspirated and turbocharged versions of its 3.0 CSL, opted to build what then head of motorsport Jochen Neerpasch calls "a race car for the road".
The BMW road-racer was the M1. It did race in Group 5 configuration, yet only fleetingly. Production issues at Lamborghini, which had been subcontracted to build the cars, resulted in delays completing the pre-requisite number of cars for Group 4. That explained the Procar one-make series on the Formula 1 undercard in 1979 and '80.
By the time, the car was homologated, BMW had bigger fish to fry. It was heading for F1 with Brabham and a successful association spanning seven seasons that would yield the 1983 F1 world title for Nelson Piquet.
Group 5 wasn't only about German manufacturers, though the importance of the DRM German racing championship gave them an extra incentive to build cars. Lancia was another willing participant with its Beta Montecarlo, winner of the WCM in 1980 and its successor, the World Endurance Championship, in 1981.