 The DH.9 was a disappointment owing to its under-performing and unreliable engines, and the DH.9A was to use a more powerful engine to resolve this.As the Rolls-Royce Eagle engine used in the successful DH.4 was unavailable in sufficient quantities, the new 400 hp (298 kW) American Liberty engine was chosen instead.As Airco was busy developing the Airco DH.10 twin-engined bomber, detailed design was carried out by Westland Aircraft.