The 331 Series Auxiliary Power Units (APUs) provide high levels of reliability with economical maintenance and have the highest power-to-weight ratio in their class. The 331 APU Series can be found on many wide-body applications for both Boeing and Airbus.

The 331-200 introduced several new technologies to the world of auxiliary power. The 331-200 APU was the first electronically controlled APU and the first to employ the same internal design as an existing turboprop engine, dramatically reducing development time and manufacturing costs.

The 331-500 series became the first Honeywell APU to use an eductor for oil cooling, eliminating the need for oil cooling fans. The success of this design led to its use on both the Honeywell 131-9B and RE200 series.