On 15 June 1951, the Board decided to resume participation in racing events from 1952 on, and gave the final orders for the construction of the “300 Sports Light”, as the new vehicle was designated. Later on the suffix was abbreviated – giving the combination SL for the model series. Its M 194 engine was derived from the M 186 engine of the Mercedes-Benz 300 type luxury saloon and the elegant two-door 300 S, having an inclined separation plane between cylinder-head and engine block, an overhead camshaft, large intake valves, combustion chamber in engine block and pistons, a displacement of 3 litres and an output of 115 bhp (85 kW). For its use in the racing car the engineers increased the engine’s output to around 170 bhp (125 kW). The sports engine differed from the one installed in the saloon and coupé not only in its output, but also in its installation position, slanted 50 degrees to the left, and in having a dry-sump lubrication system, which due to the absence of the oil sump, enabled a lower installation height.

Weight savings were hardly possible with the engine and the transmission of the W 194 that was being created. And this was also true of the heavy steel axles also taken from the 300 model. There thus remained only the frame and the exterior skin for any possible weight-savings. Another possibility for enhancing competitiveness was to create a body as aerodynamic as possible. Rolf Uhlenhaut, at that time Head of passenger car research at Daimler-Benz, took up his idea of a lightweight tubular frame again, an idea he had entertained some years before. The designers then carried the concept forward to its logical conclusion, creating a lightweight, extremely torsionally rigid frame consisting of very thin highly-alloyed steel tubes assembled to form triangles, whose tubular elements were only subjected to tensional and compressive forces.

The entire frame weighed just 50 kilograms and became the backbone of the W 194. The frame of the 300 SL (W 198), the production sports car presented in 1954, was also derived from this frame.

The coachbuilders in Untertürkheim and Sindelfingen spared no effort with the vehicle aluminium body. Thanks to the canted position of the engine and the aerodynamic profile they strove to create, the car was very low, free of trim right down to the underbody, with an elegant low bonnet line, intuitively round-shaped, with recessed headlamps and wheels entirely covered by the bodywork. The classic Mercedes-Benz radiator shape was replaced by a flat racing car front end analogous to that of pre-war cars. The Mercedes star dominated the radiator grille prominently. The passenger compartment was made as narrow as possible, with a strongly raked windscreen, curving round towards the A-pillars. The large rear window flowed over into the aerodynamic rear end. The result was a relatively small frontal area: 1.8 square metres. A drag coefficient was measured on a 1-in-5 scale model and found to be cd = 0.25, a very good value indeed for those days, and that, even without considering the realistic airflow through the engine compartment.

The doors are a chapter all of their own: in order to bestow on a space frame the desired highest-level rigidity, it has to be as wide as possible in the passenger cell sector. This requirement led to the spectacular and later so famous gullwing doors. These doors, deeply incut into the roof, opened upwards and reached only down to the waistline, resembling hatches more than doors. In order to facilitate access over the high side sill, the bodywork designers had even originally intended to have an access step in the lower part of the vehicle body flank; however, this feature was never realized. At the time, the FIA regulations did not specify the type and direction of opening doors.

In spite of this, the stewards got very flustered when the vehicle was presented to them for scrutineering before the Mille Miglia in May 1952. In the run-up to the 24 Hours of Le Mans the doors were extended down into the car’s flanks – to forestall any future protests – thereby assuming their final shape. Now they looked even more like outspread wings, for which reason the car was dubbed “gullwing” by the North Americans and “papillon” (butterfly) by the French. In two races, by the way, the 300 SL participated not in the guise of the “gullwing” car, but with a roadster body.

The interior was fully padded and lined, radiating a level of comfort unusual for racing cars. Speedometer and rev counter were located in the optimum position in the driver’s field of vision, below them and in somewhat smaller format were the gauges for water temperature, fuel pressure, oil temperature, and oil pressure. Even a stopwatch was installed. The bucket-type seats with high side sections were covered with tartan-style woollen fabric; the four-spoke steering wheel was removable to facilitate climbing in.

The archetype of the 300 SL, chassis number W 194 010 00001/52, completed its first test drives in November 1951, on the Solitude racetrack on the outskirts of Stuttgart, on the Nürburgring and on the Hockenheimring. The Mercedes-Benz 300 SL racing sport coupé, unusually smooth and, at a height of just 1,225 millimetres, exceptionally low, was presented to the excited and stunned press on the motorway between Stuttgart and Heilbronn on March 12, 1952.

A total of ten W 194 series cars were built for the 1952 season. After the Le Mans race, it was planned to enter the SL in a sports car race on the Nürburgring. In order to make the cars intended for this race as light as possible, they were given a roadster body; a further, narrower car had been set up as a roadster right from the start.

To permit the driver easy access, the section of the door extending into the side of the car was retained, and a small windscreen was mounted to deflect air and flying insects. This resulted in a weight advantage of 100 kilograms over the coupé.

The year 1952 was an extremely successful one for the Mercedes-Benz racing department: the results of that year for the 300 SL: second and fourth places in the Mille Miglia; triple victory in the Sports Car Grand Prix of Bern; double victory at the 24 Hours of Le Mans; quadruple victory in the sports car race on the Nürburgring. The last big adventure of the season was participation in the third Carrera Panamericana Mexico, an arduous long-distance race over five days and eight stages, 3,100 kilometres across Mexico. Mercedes-Benz accepted the challenge and entered two coupés and two roadsters in this rally, all powered by engines with an output that had meanwhile been boosted to 180 hp (132 kW). Karl Kling with Hans Klenk and Hermann Lang with Erwin Grupp scored a legendary double victory for Mercedes-Benz in November 1952 in this contest.

The 300 SL was reengineered for the 1953 season. The eleventh SL manufactured received the number W 194/11. Its bodywork was now entirely made from magnesium sheet, even lighter than aluminium. In the wind tunnel it gained – especially in its front section – not only a new face, but also a better airflow through the engine compartment thanks to an optimised shape. The engine output rose, too, among other things thanks to petrol direct injection, which boosted the power of the six-cylinder unit to 215 bhp (158 kW).

The rear axle was further developed to a low-pivot single-joint swing axle, while the transmission was flanged on the rear axle following the transaxle principle, which made for a more balanced weight distribution. The wheelbase was shortened by 100 millimetres. The vehicle stood firmly on 16-inch wheels, and the use of disc brakes was considered.

This further-developed 300 SL – in-house affectionately called “carpenter’s plane” on account of its front end – did not actually get to race. However, its bodywork with its angular radiator, compact dimensions and ventilation fins as well as its engine with direct petrol injection already showed the genes that were to make the 300 SL production sports car of 1954 so unique. This production sports car was to become the dream sports car of the 1950s and in 1999 was voted “Sports Car of the Century”.