As a result , it was decided to base the Merlin 60 powered PR aircraft on the MK IX airframe .
At first , production of the PR Mk XI was delayed because development of the Mk VII and VIII series was delayed .
It was decided to base the Merlin 60 powered PR aircraft on the MK IX airframe .
At first , production of the PR Mk XI was delayed because development of the Mk VII and VIII series was delayed .