French urban operators have always been interested in bringing bus floor levels down, and way back in the 1960s efforts to produce a new bus with a lower floor resulted the appearance of the Saviem SC10. In 1979, the exercise was repeated, and a specification was set out for a future urban single-decker. This would have a floor level throughout the bus of only 550mm. Renault began working on their 'bus of the future', however it was to be 1985 before prototypes of the Renault R312 appeared.

Just as the need to lower the floor resulted in the SC10 having its engine placed in an unusual location, the designers of the R312 also had to think carefully about where to place the engine in order to keep the floor level down. Eventually they settled on the idea of positioning a vertical engine across the back of the bus, in a similar way to a British rear-engined double-decker. This layout removed the need to ramp the floor up at the back of the bus in order to clear a rear-underfloor mounted engine, and therefore left space for a large standing platform and exit in the rear overhang. However it also meant that the bus had to be longer (a full 12 metres) to compensate for the space taken up by the engine compartment. Using slightly smaller wheels than normal allowed further reductions in floor height.

As with the SC10, RATP were a major force behind the development of the new bus. In fact, all of the nine prototype buses were bodied in RATP's central workshops. Only two of these actually entered the RATP fleet for test operation on Paris bus routes. A further bus was kept by RATP for experimental work, while the remainder were scattered around various provincial towns for evaluation. The six recipients of the prototype buses were Le Havre, Angoulême, Strasbourg, Toulouse, Marseille and Lyon. Some of the recipients, such as Lyon, became faithful R312 customers, however others, including Marseille and Toulouse, were evidently not impressed and failed to buy any production R312s at all. A number of big city operators avoided the R312 due to its complicated driveline, and chose to stick to more conventional machinery.

Despite any misgivings about its complex mechanical layout, the R312 went into full production in 1987. As expected, RATP proved to be a major customer and almost 1600 were eventually delivered for service in the capital. Provincial orders were of course much smaller, but the type soon found its way into many urban fleets. There were some export sales, to Belgium, Spain, Switzerland and (somewhat surprisingly) Germany.

At the time of its introduction the R312 seemed quite big for an urban single-decker, although there's more than enough power to move the large frame around! The engine is the turbocharged version of the Renault MI-series, and was therefore already familiar to operators thanks to its use in the PR100-series. Renault's bodywork for the R312 is rather boxy in profile and has big, deep windows to match the low floor. Together with the low-profile tyres, this conspires to make the R312 look very big and square indeed. The vertical rear engine results in a very small, high-set rear window.

On the inside, the bus is equally distinctive. The big windows make the interior seem very spacious, and the small rear window allows enough light in to prevent it from seeming dark at the back of the bus. The central gangway is flat throughout the bus with large standing areas opposite the exit doors. Many of the seats are cantilevered out from the sides of the bus to allow the space under them to be cleaned easily – all the seating is on raised platforms either side of the central gangway. In the cab, most of the controls are mounted on an unusual-looking console attached to the adjustable steering column, apart from the speedometer which sprouts out of the dashboard – the console idea hasn't been carried forward to newer models, perhaps because it tends to rattle in sympathy with poor road surfaces.

The vast majority of R312s are three-door buses, with entrance at the front and exits in the centre and at the rear. From early on, Renault promised a two-door version, although it was some years before any were actually made. The two-door variety (which has a single, central exit placed one bay farther back than on the three-door version) is far less common than the three-door, although they can be seen in some numbers in Paris where RATP bought several hundred of them.