The prototypes were used to test various adaptions and potential roles for the aircraft. "LV626", the first prototype, was rebuilt to the "C.II standard", the principal modification of which was the installation of Bristol Hercules VI radial engines in place of the Merlins; it was later decided to standardise on the Merlin engine, leaving this as the sole Hercules-powered York. The fourth prototype, "LV639", was furnished as a paratroop transport, complete with ventral dropping doors. However, flight testing found that the York was unsuited to this role, due to the slipstream wash drawing the parachutes towards the fixed tailwheel, posing an entanglement risk.