For 1934, Grand Prix racing's governing body introduced a formula restricting car weight to 750 kg without driver, fuel, oil, water, or tyres — a significant reduction from the heavy machines of the previous era. Daimler-Benz had been running the outdated seven-litre Mercedes-Benz SSK, developed in the 1920s under Ferdinand Porsche, who had since left to found his own firm. Porsche proposed his P-Wagen mid-engine concept to Adolf Hitler, who in early 1933 announced state funding for two competing German racing programmes: Mercedes-Benz and Auto Union, which had taken over Porsche's design.
Mercedes received an annual stipend of 250,000 Reichsmarks to develop the W25, joining a state-backed rivalry with Auto Union that would define Grand Prix racing for the remainder of the decade. The two German teams, both running silver cars — earning the collective Silver Arrows name — dominated the sport until the outbreak of World War II in 1939.
The W25 used a supercharged straight-eight engine designated M25, which Daimler engineers developed to produce more than 300 hp from the outset despite the formula's weight restrictions. The car's development continued to expand the engine's capacity and output over subsequent seasons. The M25C variant used in 1935 displaced more than four litres and produced well over 400 hp.
Development through 1935 and 1936 led to increasingly strained engine reliability as capacity grew toward 4.7 litres and 490 hp, prompting engineers to test a V12 alternative. The V12 proved too heavy, and to accommodate it the chassis was lightened and shortened — designated W25K (K for kurz, meaning short) — but this compromised handling. Young engineer Rudolf Uhlenhaut, who drove the car himself during development, determined that the W25 suffered from a too-stiff suspension in a too-flexible chassis, a diagnosis that led directly to the design of the W125.
The W25 made its competitive debut at the 1934 Eifelrennen at the Nürburgring, where Manfred von Brauchitsch won. This race is often cited as the origin of the Silver Arrows name, though a silver streamlined SSKL driven by von Brauchitsch had already attracted similar descriptions at the 1932 AVUS race.
In 1934 the W25 won four major races — the Eifelrennen, Coppa Acerbo, Spanish Grand Prix, and Italian Grand Prix — compared to three for Auto Union. The 1934 French Grand Prix was a disaster for both German teams, with none of their cars finishing. The Belgian Grand Prix also proved embarrassing when Belgian customs officials demanded heavy duties on the special alcohol-blend race fuel, prompting both teams to withdraw.
Auto Union's Hans Stuck, competing with the mid-engine Type A, proved particularly effective in hillclimb events where the rear-engine car's traction advantage told.
The 1935 season brought the revival of the European Championship after a hiatus, and the W25 dominated. Rudolf Caracciola won the championship, claiming three of five championship events to take the title. Luigi Fagioli also contributed victories, with the W25 accounting for nine Grand Prix wins across the season.
The W25 used in 1935 was sometimes designated the W25B, reflecting engine development. Auto Union continued to win races — Varzi, Stuck, and the emerging Rosemeyer all took victories — but Mercedes' pace in championship races gave Caracciola the decisive advantage.
The W25K of 1936 was less successful, its handling compromised by the chassis modifications needed to accommodate engine development. Caracciola managed two victories, most notably at the wet 1936 Monaco Grand Prix where circuit characteristics reduced the impact of Auto Union's power advantage. Facing the superior Type C from Auto Union, Mercedes chose to withdraw from several rounds to focus on development of the W125.
Two W25K cars with streamlined bodies powered by V12 engines were entered at the 1937 Avusrennen, where the banked north turn made weight and handling less critical than outright speed. Streamline variants also appeared in record attempts on public roads.
The W25 established Mercedes-Benz as a force in 1930s Grand Prix racing, providing the foundation — technically and commercially — for the more advanced W125 and W154 that followed. The car achieved a cultural afterlife in an unlikely context: as part of a partnership between Mercedes-Benz and Nintendo, the W25 Silver Arrow was included as a playable vehicle in Mario Kart 8 and Mario Kart 8 Deluxe, introduced as downloadable content on August 27, 2014.