 The Mk I was faster than the Spitfire Mk VC at and faster at , despite the British aircraft's more powerful engine (the Merlin 45, producing at .Although it has often been stated that the poor performance of the Allison engine above was a surprise and disappointment to the RAF and USAAF, this has to be regarded as a myth; aviation engineers of the time were fully capable of correctly assessing the performance of an aircraft's engine and supercharger.As evidence of this, in mid-1941, the 93rd and 102nd airframes from the NA-91 order were slated to be set aside and fitted and tested with Packard Merlin engines, with each receiving the designation XP-51B.