The bus - the Enviro500, unveiled in 2002, is a 12-metre (40 foot) tri-axle double-decker bus with a Guildford-built chassis and Falkirk-made body designed to carry more than 120 people. The vehicle exceeded some design limitations in Hong Kong, prompting the local Transport Department to make some exclusions in order to make the buses available to KMB on time.

The width of the bus is 2550mm (100.3 inches), exceeded the limit of 2500mm (98.4 inches). The width of 2550mm, approved by the EU, became the current standard. A fixed glass replaced the emergency door at the rear of the upper deck, but with more hammers available for emergency. The front indicators are the smallest of all current models of KMB. The modified version of Enviro 500 body, first built in late 2003, have revised frontal and rear designs.

The Enviro500 is the first model of second-generation low-floor buses for Kowloon Motor Bus, with straight staircase and plug door (only at the exit) fitted. All KMB's Enviro500 buses had a bronze band added to the champagne livery to differentiate them from the first-generation low-floor buses.

When TransBus announced the development of the Enviro500, KMB became the launch customer by switching the last 20 of an order of 100 Dennis Trident 3 to the Enviro500 in 2002. Later in the same year, KMB placed a second order for a further 100 of the vehicles. The first Enviro500 arrived in Hong Kong on 12 November 2002 and was registered in January 2003. The remaining 119 buses entered service between March 2003 and November 2003.

In 2003 KMB placed the third order for 65 Enviro500s, quickly followed by a further 50. The bodywork of these buses received a number of minor modifications. The first 112 buses entered service between November 2003 and June 2005, and the remaining three buses were registered by its subsidiary Long Win Bus.

Due to the commencement of the KCR West Rail in late 2003 and KCR Ma On Shan Rail in late 2004, Transport Department of Hong Kong requested KMB to reduce its fleet size, so the registration of these four batches of KMB Enviro500 were delayed and some of them were stored for a long time before entering service.

In 2005, KMB placed a further order for 25 more vehicles with modified bodywork and new Alexander Dennis badge, 24 of them (at least five had their electronic route destination displays supplied by Gorba instead of Hanover) entered service in early 2006, and the last one, a Euro IV-engined prototype, with fleet number ATEU1 (originally ATE257), has entered service on 24 May 2006. The bus is one of the two buses which are first to meet the Euro IV emission standard in Hong Kong. (Another one is a Volvo B9TL with fleet number AVD1.)

A new order for 15 more buses was placed by KMB in late 2005; they entered service in August 2006.

ATE33 entered service with KMB in May 2003