In 1954 Mercedes-Benz unveiled its first semi-integral bus – the O321 H. The semi-integral design meant a reduction in weight, improvements in stability and body resistance. The O321 H also was the first to feature coil springs in the front-axle suspension. This 9.2-metre-long vehicle (a 10.9-metre version was later unveiled) also featured a rear-mounted engine. The first version was available with an output of 110 hp, and a later optional 126-hp version was made available. More than 30,000 units of the O321 H complete bus and its platform were sold around the world, a mark which places it as the best-selling bus of its time and, until today, one of the most successful models by Mercedes-Benz.