Rudolf Uhlenhaut, design chief at Daimler-Benz, was responsible for both the W 196 R Formula One car and the W 196 S sports racer that grew from it. The 300 SLR designation stood for Sport Leicht-Rennen — sport, light, racing — later condensed to SLR. The designation connected it commercially to the 300 SL name but the two cars shared no engineering ancestry; the SLR was an enlarged development of the Grand Prix car.
The engine, designated M 196 S, was a naturally aspirated 2,982 cc straight-eight with a bore and stroke of 78 mm. It produced a peak output of 310 PS at 7,400 rpm, sustained racing power of 276 PS at 7,000 rpm, and maximum torque of 311 Newton-metres at 5,950 rpm. Like the Formula One unit, the engine was canted 53 degrees to the right to lower the car's centre of gravity and reduce frontal area. Power was taken from the centre of the crankshaft via a gear rather than from either end, reducing torsional stress. Desmodromic valve actuation and Bosch direct fuel injection completed the specification. The engine was designed to run on commercially available Super 98 RON petrol.
The chassis was a brazed steel tube spaceframe carrying bodywork made from Elektron, a magnesium-alloy with a density less than a quarter that of iron. The roadster weighed 901 kg. A second seat was provided for a co-driver, mechanic, or navigator. The cockpit layout used for the W 196 R monoposto configuration was widened to standard two-abreast seating, and headlights were fitted for endurance events. Nine W 196 S chassis were built in total: seven roadsters and two closed coupés.
Mercedes entered the World Sportscar Championship selectively in 1955, focusing on the European calendar. The season opened with an unforgettable performance at the Mille Miglia, a race run on public roads over approximately 1,600 kilometres of Italian terrain. Stirling Moss, navigated by journalist Denis Jenkinson using hand-written pacenotes, won at an average speed of 157.650 kilometres per hour — a record for the event. Juan Manuel Fangio finished second in a sister car. The result demonstrated the car's extraordinary combination of speed and reliability.
At Le Mans in June, Daimler-Benz fitted the 300 SLRs with large hinged air brakes above the rear deck, designed to reduce wear on the inboard drum brakes during deceleration from the circuit's long Mulsanne Straight. The addition addressed a genuine weakness: the disc-braked Jaguar D-type posed a significant braking advantage. During the race, Pierre Levegh's 300 SLR became airborne after contact and crashed into a spectator area. The ultra-lightweight magnesium bodywork ignited on impact. Eighty-three spectators and Levegh lost their lives in the accident, which remains the highest-fatality crash in motorsport history. The leading Mercedes cars were withdrawn overnight.
Despite the Le Mans withdrawal, Mercedes continued in other championship rounds. The 300 SLR scored a 1-2-3 finish at the Tourist Trophy at Dundrod, Ireland, and another 1-2 at the Targa Florio in Sicily. These results, combined with earlier points scored, were sufficient to secure the World Sportscar Championship for Mercedes by two points ahead of Ferrari. Counting non-championship races, the 300 SLR won five events in the 1955 season.
Mercedes-Benz had already decided before Le Mans to withdraw from motorsport at the end of 1955 to concentrate resources on new road car programmes. The Le Mans disaster reinforced that decision. The company would not return to major factory competition for three decades.
Before the Le Mans accident occurred, Rudolf Uhlenhaut had commissioned two of the nine chassis to be set aside and converted into road-legal closed coupés, intended for the Carrera Panamericana. The resulting cars featured gull-wing doors, necessitated by the spaceframe's high sill structure, and had maximum speeds approaching 290 kilometres per hour — making them, by a large margin, the fastest road-legal cars in the world at the time.
When the Carrera Panamericana was cancelled following the Le Mans disaster and Mercedes announced its motorsport withdrawal, the coupé project was abandoned. Uhlenhaut retained one of the completed cars as a personal company vehicle, fitted only with a large silencer to reduce the near-unsilenced exhaust. Stories circulated that he used it to cover the 220-kilometre Munich-to-Stuttgart autobahn journey in just over one hour. The other coupé was preserved in the Mercedes-Benz Museum.
In May 2022, the museum coupé was sold by RM Sotheby's at a private auction held at the Mercedes-Benz Museum for €135 million, becoming the most expensive car ever sold at public auction or private sale. The proceeds were used to establish the Mercedes-Benz Fund.
The 300 SLR is credited with inspiring the Mercedes-Benz SLR McLaren grand tourer developed jointly with McLaren Automotive, which debuted in 2003. More broadly, the 300 SLR represents the apex of Mercedes-Benz's postwar return to racing — a brief, dominant, and ultimately tragic campaign that established the Silver Arrows' reputation as the most advanced constructor of the era before circumstances ended it entirely.