The P-51 was not king. There were many capable aircraft on all sides during the war, many out-performing the P-51 in some aspects of their performance envelop. The P-51 just appeared at the right time with the right performance that allowed it to make a significant impact to the war. When I first joined this site seven years ago I did not know much about the P-51 even though I had been an AF pilot. My favorite aircraft was the F4U and Navy aircraft in general. I had Naval aircraft models hanging all over my room (my favorite aircraft is still the F8U). However, it appeared to me that at that time, there was quite a lot of P-51 bashing going on, some justifiable due to its overly glowing popularity. I decided to put on the mantle of nationalistic pride and become a defender of the Mustang, and started studying this aircraft. The more I studied, the more I was impressed with this quite capable aircraft.
There are significant myths still accepted by many, including members of this site. One of these myths is that the Mustang was an average fighter that was successful because of its range and its numbers. Another is that it while it was a capable medium-to-high altitude fighter; it lacked capability at low altitude. Third, the P-51 only was effective because it always had significant advantages in numbers compared to the opposing enemy defenders. Let me address the first two issues as the third has often been proven wrong in several places on this forum.
First, the P-51 was fast. With its clean aerodynamics, already discussed by Drgondog, it was faster than the great majority of contemporary enemy aircraft from SL to ceiling, mostly, significantly faster. With a SL speed of over 380 mph (386 mph for the B), there were no aircraft, except for the Tempest II, significantly faster until the final generation of propeller aircraft. At high altitude where it was in its element, only the much too-late and too few, hot-rod, short-ranged Bf-109K-4, was faster.
Second, it could climb with the best of them. After May, 1944, the P-51B had a SL climb rate of over 4400 ft/min., clean and at fighter weight. Again only the Bf-109K-4 was significantly better at climb rates in general.
The late 1944, Fw-190D-9, an excellent propeller fighter considered by many as the best German propeller fighter of the war, and the considered by many to be the best WW2 propeller fighter, could only fly nose-to-nose in speed and climb with the mid 1944, P-51B up to 15-20k ft. Above that, the P-51 quickly outperformed it in speed and climb. The P-51D was only slightly behind the two below 20k, outperformed the Fw-190D-9 above 20k. The Fw-190D-9 did have better firepower than either P-51s, but the P-51s certainly demonstrated effective firepower against fighter aircraft. In addition, the P-51 empty weight was 7,125 lbs with a max TO weight of 11,600 lbs for a total loaded weight capacity of 4475 lbs. The Fw 190D-9 had an empty weight of 7,694 lbs and a max TO weight of 10,670 lbs or a total loaded weight capacity of 2,976 lbs., 1500 lbs less than the P-51, which makes the P-51 far more flexible in application.
As was mentioned in another entry, the P-51 has the cleanest airframe for any WW2 propeller fighter. Now let's look at the effects of aerodynamic efficiencies on climb and acceleration. Climb and Acceleration is a function of excess power, not absolute power. Comparing the post May '44, P-51D with the Bf 109G-14 (ASM), a powerful machine. These aircraft are roughly equal in power At 20k ft but the P-51 is approximately 40% heavier in empty weight. At this altitude, the max speed of the Bf 109 is 403 mph. The max speed of the P-51 is 431 mph. If these two aircraft were flying side-by-side at that airspeed, the Bf 109 would be balls-the-wall using all its available power to maintain airspeed. The P-51 on the other hand, would be throttled back to about 80% (if my estimate is accurate). So, at these conditions, the P-51 can out-accelerate the Bf 109 because the Bf 109 cannot go faster without descending whereas the P-51 can easily do so. Similarly, the P-51 can out-climb the Bf 109 because the Bf 109 has no excess power to climb at that airspeed whereas the P-51 has excess power to do so. As airspeed goes down, this ratio does change but the concept remains in effect. The point is that simple wing loading and power loading values are only part of the equations for climb and airspeed and helps explain why the relatively heavy P-51 is such a good performer.
The P-51 was not the king, but it was an exceptionally good fighter and made a major impact on the war. It was a fighter that could fly 600 miles, fight viciously at all altitudes, and was competitive to enemy aircraft over the enemy homeland, and then fly back. While noted as an excellent escort fighter, it is often overlooked in its devastating ability as a deep interdiction fighter and is often disrespected for its very good dog-fighting capability at all altitudes. It was an unusual plane in that while it had exceptional capability in speed and range, it had very few weaknesses.