The fundamental design originated with Robert Eberan von Eberhorst and was completed by his protege and successor David Hodkin as the chassis for the ERA G-type Formula Two car — Leslie Johnson's attempt to revive the ERA name in 1952. The G-type was initially planned around a custom ERA engine, but funding difficulties led to adoption of the Bristol straight-6 unit, which had also been used with success by Cooper, Frazer Nash, and AC. Engine modifications by Hodkin resulted in failure in all three of the G-type's Grand Prix outings, despite the car being driven by the rising Stirling Moss. By the end of 1952, Johnson, out of money and in failing health, sold the project to the car manufacturing arm of the Bristol Aeroplane Company.
In late 1952 Bristol announced it would redesign the car as a two-seat sports car for Le Mans. The G-type was comprehensively rebuilt by Bristol's new motorsport department under design team leader David Summers. The chassis used twin oval-section rails running the full length of the car, changed to circular-section members for the Bristol. The G-Type's double wishbone front and de Dion tube rear suspension were retained. The wheelbase was relatively long at 8 ft 1.25 in (247 cm) given the car's narrow track of 4 ft 3 in (130 cm). Inboard rear brakes and an innovative hub and rim arrangement reduced unsprung weight; the large hub spiders allowed driveshaft changes without removing wheels or brakes. For 1955, Bristol developed a powered multi-barrelled spanner that could remove all five wheel nuts simultaneously during pit stops.
The tall Bristol straight-6 was mounted forward, driving via propshaft to a four-speed gearbox behind the driver, achieving near-even weight distribution. Fuel was carried in twin pannier cells behind the front wheels. Engine modifications for 1954 raised output from approximately 140 bhp to 155 bhp (116 kW), lifting top speed to over 140 mph with Le Mans gearing. Kerb weight of only 1,450 lb (660 kg) gave a power-to-weight ratio exceeding 235 bhp per tonne. Driver Lance Macklin remarked that the Bristol 450 "could be driven without hands."
As a division of an aircraft manufacturer, Bristol had access to a company wind tunnel and designed the car for aerodynamic efficiency. The original bodies featured smooth shapes with pronounced headlamp and air intake bulges and distinctive large twin stabilising fins running down the rear of the enclosed cockpit to the tail tip. After the debut race both cars returned to the factory damaged. During repairs the styling was substantially revised: panelwork was smoothed further, small front wings removed, headlamps recessed into faired conical tunnels, and ducted openings behind the front wheels added to improve brake cooling. For 1955 the cockpit was redesigned to open-top configuration; wind tunnel work showed that the reduction in frontal area outweighed a slight increase in drag coefficient. A single large tailfin replaced the twin fins and a solid tonneau covered the compulsory second seat, in a layout closely resembling the 1955 Le Mans-winning Jaguar D-Type.
The Bristol 450 made its race debut at the 1953 24 Hours of Le Mans with two cars entered and a third held in reserve as a spare. Both retired before half distance when crankshaft balance weights detached, locking the rear wheels at speed and causing both cars to leave the road and catch fire. Despite the retirements, Jack Fairman had set a new lap record for the 2-litre class. Three weeks later the rebuilt cars, now wearing revised aerodynamic bodywork, entered the 12-hour race at Reims. One retired on the first lap with transmission failure, but the other took a class victory and finished fifth overall.
In October 1953, Bristol returned to France for endurance speed record attempts at Montlhery. On 6 October the team set six records for the 2-litre class: 200 miles at 125.87 mph; 500 km at 116.10 mph; 500 miles at 112.25 mph; 1,000 km at 115.49 mph; 3 hours at 116.42 mph; and 5 hours at 115.43 mph.
At the 1954 Le Mans race all three entered cars finished, taking first, second, and third in class and seventh, eighth, and ninth overall despite torrential rain, earning Bristol the team prize. The cars then ran at Reims, finishing second, third, and fourth in class behind a Ferrari. At the 1955 Le Mans race the open-bodied cars again dominated, finishing seventh, eighth, and ninth overall and sweeping the class. The lead car averaged over 100 mph for the opening 12 hours and spent only fifteen minutes in the pits over the full 24-hour race.
The 1955 Le Mans disaster — which killed Pierre Levegh and 83 spectators — combined with a corporate shift away from sporting credentials persuaded Bristol to withdraw immediately. Prize money was quietly donated to the victims' relief fund and the team withdrew from the 1955 Reims race. Rather than allow inexperienced privateers to run the cars in unsuitable events, Bristol selected the best components from all chassis, built up one complete car, and scrapped the remainder. This survivor was retained by Bristol Cars owner and former racing driver Tony Crook for thirty years, occasionally loaning its engine to road cars. It was fully restored to 1955 specifications in the late 1990s and appears infrequently at historic race meetings.
Although most sources state only three 450s were built, photographic evidence suggests at least four were in existence at one point.