What about Japan? US heavy, and even medium bombers were very successful at driving off Japanese fighters, even when greatly outnumbered.
You have a generally reasonable point that all fighters weren't uniformly successful against all bombers they could easily catch. Depended on the characteristics of the fighters and bombers, and numbers of each. But I think your generalization about Japanese fighters v US bombers goes too far:
-in period of frequent Japanese fighter attacks v US mediums, B-25's especially were sometimes mauled by Japanese fighters; over New Guinea in 1942 the basic conclusion was unescorted B-25's were not viable v Japanese fighters. It was somewhat different for B-26 because the plane was faster, if not intercepted well before bomb release, B-26's could dive away and be pretty hard for their main opponent in that theater, the Zero, to catch. But even in that case the 5th AF recognized the need to escort medium bombers to achieve reasonable % loss rates (one basic issue comparing ETO and early PTO is number of a/c, the flight of the 6 B-25's of 3rd BG essentially wiped out by Zeroes over Lae May 25 1942 has a noticeable impact on the overall loss rate stat for 5th AF 1942, 6 B-17's downed in 1944 over Germany has almost no impact on the overall stats for 8th AF 1944). Another example is 11th AF B-25's (w/ B-24's) v the Kuriles flying from the Aleutians in 1943 unescorted: heavy losses to intercepting fighters, raids discontinued.
-later (1943-45) unescorted US medium bomber missions faced Japanese fighters generally less often. It was heavies mainly, in generally small scale raids by B-24/PB4Y, and later the early-phase B-29 raids over Japan, which flew far enough to have to routinely face Japanese figthers alone. That's not so different from ETO actually (9th AF lost 131 mediums to German fighters, 8th lost 2,452 heavies to German fighters, per USAAF stats).
-back to numbers, there were many missions in 1942 by B-17's in Pacific where small formations or single a/c avoided losses when intercepted by Zeroes. But the number of Zeroes intercepting was often overestimated, and the number of Zeroes shot down grossly overestimated. Zeroes shot down more B-17's in 1942 than B-17's downed Zeroes in reality, and 1:1 is obviously a much worse exchange rate for single v 4-engine a/c than between fighters. And the B-17 loss *rate* was not necessarily that low. Of course it also depends how much loss each side can afford to take, and morever how effective the bombing is, but unescorted raids by B-17's/24's against Japanese fighter opposition had a mixed record actually. In the more effectice cases, and this was true of mediums too, it was often against ships at sea that the Japanese fighters had a hard time covering continuously; this is once the a/c adopted effective low altitude bombing tactics, which B-24's as well as mediums did as the war went on, even B-17's did in some cases at the end of their Pacific career ca. early 1943. You can't directly compare those missions to bombing stationary strategic targets deep inside Germany,
-or to B-29's bombing Japan. In that case, unescorted day raids mainly from Nov 1944-Feb 1945, the loss rate to fighters was definitely lower than for B-17's against Reich targets in 1943 unescorted (or partly escorted). But the rate wasn't really low absolutely, and the very high altitude bombing which contributed to reducing losses to fighters (and AA) also contributed to unsatisfactory bombing results. The B-29 first became a highly successful unescorted bomber at night, in the devastating night fire raids in March 1945, then afterwards in both and night, but against rapidly diminishing Japanese fighter threat. A few later B-29 raids were escorted by Iwo Jima based P-51's, but mainly the general presence of carrier fighters, P-51's and shortage of fuel caused the Japanese to hold their fighters on the ground more and more in spring-summer 1945. Some later B-29 raids were daylight unescorted again but faced much less opposition.
Joe