The 403 was the third of the eventual five series of Bristols powered by the BMW-derived pushrod straight-six engine.

It retained much the same styling as the 401, but featured many mechanical improvements compared to that model.

The 1971 cc six-cylinder engine was modified through the use of bigger valves and larger main bearings with a diameter of 54 mm as against 51 mm on the 400 and 401, which increased the power output to 100 hp (75 kW) as against 85 hp (63 kW) on the 401.

[1] To cope with this increased power, an anti-roll bar was fitted on the front suspension and improved drum brakes known as "Alfins" (Aluminum finned) were fitted.

It is also noteworthy for having two extra headlamps at the side, almost pre-dating the adoption of the four-headlamp layout in larger cars (Bristol themselves adopted it with the 411 in the late 1960s).