The D-Series was introduced in February 1992 as MCI's first 45-foot (14 m) motorcoach, enabled by a December 1991 law change that allowed the additional length.

First, was the growing calls to allow 45-foot (14 m) coaches (at the time prohibited by US law), second was that MCI's existing models were designed to use two-stroke engines and the company was looking to offer the new Detroit Diesel Series 60 four-stroke engines, and third was 1988 Canadian government request to develop a wheelchair accessible intercity bus.

[1] On December 18, 1991, the Intermodal Surface Transportation Efficiency Act was signed into law, allowing an increase of coach length to 45-feet.

[2][3] The bus was quickly adopted by private companies operating scheduled service (like Greyhound Lines) or commuter buses.

The downside compared to a traditional low-floor transit bus is that motorcoaches only have one door and a high-floor that requires the use of a wheelchair lift, slowing boarding/alighting.

In 2022, MCI added a D-Series production line to the NFI group factory in Crookston, Minnesota due to the planned and later canceled closure of the Pembina plant.

It has an air intake covered by a large grille, a two-piece engine door, small headlight/taillights, and a black roof cap.

The rear end cap design was updated in 2007 to accommodate new engines that met stricter EPA emissions regulations.

For example" "D" for generation or basic model, "45" for the length, "CRT" for "commuter rapid transit" and "LE" for modes with the Low Entry door.

The intercity coach model, the D4520, retains MCI's traditional alphanumerical naming system, with "20", denoting the NextGen D-Series.