Bristol had introduced their first sleeve valve designs in as the 750 horsepower class Perseus and the 500hp class Aquila, which they intended to supply throughout the 1930s. Aircraft development in the era was so fast that both of these engines quickly ended up at the low-power end of the military market, and in order to deliver larger engines, Bristol developed 14-cylinder versions of both. The Perseus evolved into the Hercules, and the Aquila into the Taurus.
The first Hercules engines were available in 1939 with the 1,290hp Hercules I, soon improved to 1,375hp in the Hercules II. The major version was the Hercules VI, which delivered 1,650hp, and the late-war XVII produced 1,735hp. The Hercules powered a number of aircraft, including Bristol's own Beaufighter heavy fighter design. It was more commonly used on bombers, where it could be found on the Shorts Stirling, the Vickers Wellington, one version of the Avro Lancaster, and all later versions of the Handley-Page Halifax. It was considered to be one of the most reliable aircraft engines of the era, and was well liked by both pilots and mechanics.
The Hercules also saw use in civilian designs. It was used in the Avro York and Bristol Freighter cargo planes, the Short Solent flying boat, and the Handley-Page Hermes and Hastings. The design was also licensed for production in France by SNECMA, which saw use in the Nord Noratlas.