 The principal change on this model was the adoption of the Merlin engine in place of the Pegasus XVIII; other modifications included hydraulic and oxygen system revisions along with the installation of cabin heating and an astrodome.On 3 March 1939, "L4250", the prototype Mk II, performed its maiden flight; this had been delayed due to production delays of its Merlin X engines.Stability and balance issues were encountered during flight tests of the prototype, resulting in further changes such as the enlargement of the tailplane.