The P-51's needed an escort to and from Iwo Jima to Japan where they would either join more B-29's to escort them, or they would attack airfields or other targets. The P-51's originally were shipped from Hawaii to Guam via carrier and then flew to Saipan and then were escorted to Iwo Jima by either B-29 or P-61
The B-29 was the one that carried the vital instrumentation and navigational aids for such a long trip over the inmensity of the Pacific and more capable of leading the pack. The P-51 was the first fighter during the war with the reach and autonomy to scort bombers all the way from England to Berlin and back; it was equally capable for the task to scort the B-29's during the Japan bombing campaign.
The reason for not being in front during the transverse from Iwo Jima to Japan, was because it wasn't necessary, they were never attacked on the way there, or while returning. Also, in this manner they avoided collision with the B-29's. It was not necessary to fly that close, either in front or back of the bomber. Upon reaching the target, the P-51's formed an umbrella around the B-29 for a combat air patrol. Most of the time it was not necessary because japanese fighters did not try to intercept formations
So it was up to the Mustangs to pick up secondary occasional targets. Still, few B-29's were lost due to japanese fighter attack.
The United States Army Air Force bomber command was so surprised with the lack of opposition to bomber formations by japanese aviators, that they decided to implement a lethal doctrine. General Curtiss LeMay, after sensing the negative results of high altitude bombing due to bad weather around Japan, with bombs being tossed about by high winds, decided to shift from high altitude daylight bombing, to very low altitude night bombing. The opposite concept for which the B-29 was originally built.
While the daylight strategic campaign continued over factories and military installations, night low level bombing began over the most important cities. Armor and defensive weaponry of the bombers was removed to allow increased bomb loads; Japanese night-fighters and anti-aircraft gun defense was so feeble it was hardly a risk.
Their city buildings, made of wood and paper, caused the fires to burn out of control. The bombing on Tokyo on march 9, 1945, by 334 B-29's, which incinerated 16 square miles and killed 100.000 people, was the most destructive conventional raid, and the deadliest single bombing raid of any kind in terms of lives lost, in all of military aviation history.
