115012a0_e_Raiders___First_Joint_Action__3
[0] Bombing of Japan [1] Good visibility over the target. Tail winds. [2] A low-level bombing attack was planned. Since the planes were to be launched at near maximum range, fuel consumption was vital. [3] 
Essential if the raid were to be effective. Desirable in order that reserve fuel would place bases in China within range.
Doolittle Raiders - First Joint Action
3