Iso S.p.A. in Bresso was already well known for producing the extremely well built and fast Iso Rivolta IR300; a sleek looking 2+2 Coupe based on a Chevrolet Corvette power train. After leaving Ferrari, in 1961 Giotto Bizzarrini set up “Prototipi Bizzarrini” in Livorno, Tuscany where he designed and consulted for marques like ATS, Lamborghini and Iso Rivolta. In 1963 he designed the Iso Grifo A3/L (L for Lusso {Italian for Luxury}) for Renzo Rivolta, who was looking for a follow-up to his Iso Rivolta GT. The body was designed by Giorgetto Giugiaro at Bertone, while Bizzarrini put his expertise in the mechanicals. Bizzarrini figured there would also be a demand for a race version of the Grifo and came up with the A3/C (C for Corsa) with a dramatic modified alloy body. He later dubbed it his “Improved GTO", as he had been the designer for the 250 GTO when he had worked for Ferrari. The engine was moved back about 40 centimetres (16 in), making the A3/C one of the first front-mid-engined cars ever built. To adjust the timing a piece of the dash was removed. Both cars were being built simultaneously. When leaving the factory both the 250 GTO and Iso Grifo originally fitted Pirelli Cinturato 205VR15 tyres (CN72).
That same year Bertone showed the Grifo A3/L prototype at the Turin Auto Show, while Iso showed off the (partly unfinished) competition version; the Iso Grifo A3/C. Both were overwhelmingly successful. Although design changes had to be made to the prototype, Iso concentrated on getting the Grifo A3/L ready for production. The car got a light facelift that made it less aggressive but turned it into possibly the most elegant-looking Gran Turismo (GT) supercar ever produced. This “street” Iso Grifo GL received the fast, modified but reliable Chevrolet Corvette’s 327 V8 (5.4 L) engine—either in 300 or 350 hp—coupled to a Borg-Warner 4-speed toploader. These engines were ordered in the United States, but were completely taken apart and blueprinted before they were put in; as they did with the Iso Rivolta GT. With over 400 horsepower (300 kW) and a weight of less than 2,200 pounds (1,000 kg), the vehicle was able to reach speeds over 275 km/h (171 mph).
While Renzo Rivolta focused on the A3/L; Giotto tried to promote the A3/C—making for some tension between the two. This meant that the Grifo GL was being produced at Bresso and the A3/C at Piero Drogo’s Sports Cars of Modena under Giotto’s strict supervision. In 1964 the prototype Grifo A3/C raced at Le Mans (Edgar Berney/Pierre Noblet), running well until brake problems required a two hour pit stop. The car then resumed the race, finally finishing in 14th place; an encouraging result for a brand-new car. Only 22 examples of the Bizzarrini Grifo A3/C were constructed before a disagreement between Renzo and Bizzarrini ended the cooperation.