Towards the end of the year, a full mock-up of the design was assessed; production of a pair of "HP57" prototypes commenced in March 1938. Further design modifications resulted in the definitive aircraft, now considerably enlarged and powered by four Rolls-Royce Merlin X engines. Such was the promise of the new model that, in January 1938, the RAF chose to place their first production order for the type, ordering 100 Mk.I Halifaxes "off the drawing board", at which point the serials which had already been assigned to HP56 were switched to HP57.