The Daimler Fleetline was the second rear-engined double-decker bus chassis to be launched by a UK manufacturer, following Leyland's introduction of the Atlantean in 1958. From the outset, the Fleetline had a drop-centre rear axle fitted as standard, enabling low-height bodywork to be fitted without necessitating an inconvenient seating layout in part of the upper deck, as was the case with early Atlanteans. Leyland responded by offering a drop-centre rear axle as an option on the Atlantean, but after the two companies came under the same ownership in 1968, the low-height Atlantean option was discontinued.
The prototype Fleetline was fitted with a Daimler engine, but when production started only Gardner 6LX or 6LW engines were offered. By 1968 Gardner's new and more powerful 6LXB was also an option, and in 1970 Leyland's O.680 engine became available. Gardner engines had an excellent reputation for reliability and economy while Leyland engines were more lively and more thirsty. Most Fleetline customers preferred Gardner engines, but the Leyland engine became popular - particularly for a period in the 1970s when Gardner could not meet demand.
In late 1960s, Daimler developed the longer 36 feet double-deck Fleetline. This chassis had a longitudinally-mounted Cummins V6 engine, same as the single-deck Roadliner, at the rear offside corner. It was designed mainly for export, but one was built for Walsall Corporation Transport.
In mid-1970s, Leyland developed a special version of the Fleetline, known as the B20, with Leyland O.690 engine, exhausts on both sides above the engine compartment and reduced noise levels. All of these went to London Transport.
The first prototype Fleetline was unveiled in December 1960. Between 1960 and 1973, the Fleetline was manufactured in Coventry, with production then transferred to Farington.