I know performance wise the merlin/packard motor was a huge difference and made the P-51 one of the greatest prop driven fighter of all time. But i actually quite like the look of the early mustangs. They just look so easy on the eye with their futuristic clean lines and menacing stance. I know the P-51D was the pinnacle of the mustangs design. But early 51's did a stellar job of ground attack/fighter bomber missions, with quite minimal losses too considering they were not designed at all for that type of operations. But anyway, great pic!
