The angular Gandini-designed hatchback was strongly inspired by the British 1977 Reliant FW11 concept and the 1979 Volvo Tundra concept car (also designed by Bertone). It was one of the first cars to benefit from the merger of Peugeot and Citroën in 1976, sharing its platform with the more conventional 405 that appeared in 1987. Among the features that set the car apart from the competition was the traditional Citroën hydropneumatic self-levelling suspension, extensive use of plastic body panels (hood, tailgate, bumpers), and disc brakes front and rear.
The BX used the XY, TU and XU series of petrol engines in 1.4 L, 1.6 L and 1.9 L displacements (a 1.1 L engine, very unusual in a car of this size, was also available in Italy, Portugal and Greece). The 1.1 and 1.4 unit was an old Peugeot/Reanult powertrain with its roots in the Peugeot 104 and Renault 14, but the 1.6 and 1.9 was all-new and later used in many Peugeots. The class leading XUD diesel engine version was launched in 1984. The diesel and turbo diesel models were to become the most successful variants, they were especially popular as estates. It was this Peugeot influence that endowed the BX with some much needed reliability, something for which its predecessor, the GS, had a marginal reputation. This led to the BX's famous advertising slogan "Loves Driving, Hates Garages".
All engines were badged as 11, 14, 16, 19 — signifying engine size (In some countries, a weaker, 80Bhp version of the 1.6 L engine was badged as the BX15E instead of BX16). The 11TE model was seen by foreign motoring press as slow and uncomfortable.
The 1.1 L engine with engine code H1A was specially tuned for Italy, Greece and Portugal. It was fitted to the cars made from 1988 to 1993 and produced 40 kW (55 hp DIN) at 5800 rpm.
A year after the launch of the hatchback model, an estate version was made available. In the late 1980s, a four-wheel drive system and turbodiesel engines were introduced.
All BXs are known for an exceptionally smooth ride, reasonable fuel economy and potent brakes. The perceived complexity of the suspension system meant that it was often neglected, leading to expensive repairs. Maintenance was seen as expensive because the suspension spheres (the equivalent of the spring and shock absorber on conventionally-sprung cars) needed to be replaced every two to three years, although on the BX they were easy to change and a cheap consumable. On later models, the suspension piping was better protected against corrosion, and less prone to failure. That is why there are very few original series 1 BXs on the road today. Rust protection of the bodywork was much better than earlier Citroens and also better than average.