They were designed by the NZR's Chief Mechanical Engineer, A. L. Beattie and his Chief Draughtsman, G. A. Pearson to replace less powerful locomotives struggling with increasing loads on the South Island Main Trunk Railway, and in anticipation of the traffic volumes that would be created upon the completion of the North Island Main Trunk railway.

[1] The Baldwin Q had established the Pacific as the way forward for Express passenger locomotives,[1] but the C.M.E decided that greater efficiency was needed.

The Vauclain system had proved ineffective in New Zealand so the type attributed to Frenchman Alfred de Glehn was adopted.

The first eight locomotives were built at New Zealand Railways Department's Addington Workshops, the rest by A & G Price of Thames.

Initially, fifty Price built engines were allocated to the North Island the rest to the South.

[3] Fitted with an ALCO superheater as a trial when built, it was marginally more powerful than the other A class locomotives as a result despite having only two cylinders.