The assumptions in the initial question go against some of what I know.
Bombing of Germany in WW2 did severe damage to several industries, pushing back production and some military projects.
The biggest cause of production in Japan probably wasn’t due to bombing. It was due to Japan having almost zero natural resources in Japan. The submarine war against Japan in WW2 was extremely successful and during the last twelve months or so, the Japanese had very few merchant vessels left.
Also towards the end of the war, the allies massively stepped up their mining operation. There’s a quote from some Japanese general or politician I can half remember saying something like, the sea mining could have won the war alone, it was so effective.
Then there’s the fact that most of the places Japan got its resources from were captures during the last years of the war.
I also question the premise of the OP. The air campaign just as successful as the European one was at forcing Hitler to surrender.
Curtis LeMay may have been running out of targets, but the United States was still preparing for an extremely costly and slow invasion of the Japanese islands.
You can’t credit the air campaign as a “war-winning” one unless you’re lumping Fat Man and Little Boy in with the conventional bombing.
While it is often cited that German production increased during the bombing campaign, you have to keep two things in mind: 1. As per the above Wikipedia article a large part of the increase was due to Germany finally going into full war production such as multiple shifts. 2. The production increase would have been far greater if there wasn’t a bombing campaign. (A scary thought.)
Several people have pointed out that the Allied bombing campaign of Germany was poorly planned and executed. Albert Speer repeatedly wrote out how the Allies would target one industry for a while, nearly bring it to its knees, that switch to something else thereby letting the Germans recover. If they had just kept on the specific categories (esp. Aluminum), Germany would have been in far greater trouble sooner.
It appears in most respects the bombing campaign in Japan was better organized than the German. But in 20/20 hindsight some things could have been done better. The most common belief is that aerial mining of the harbors earlier would have been the most effective thing prior to 1945.
Agree with the main points made by so many above …
Something not yet mentioned:
The main body of the air campaign against Germany proper was ballpark mid '43 to early '45. The main air campaign against Japan proper was late '44 to late '45.
As Germany & Japan were each running low on resources, factories, etc., the US was really just getting up to full steam on industrial production. As well, the rate of technological change was really amazing. The difference in combat effectiveness between the B-17D and the B-29D was night and day even though they were employed in large numbers only a couple years apart.
Japan faced a very different and rapidly strengthening USAAF than Germany did.
The air war was about far more than bombing success. D-Day could not have succeeded without air superiority. There’s also photo-reconnaissance. And maybe you’ve heard of this little thing called The Battle of Britain.
Yes, and the Battle of Britain was where German strategic bombing failed due to British air defense. The question isn’t “was it a waste of time to build airplanes”. The question is the effectiveness of the strategic bombing campaigns. There was a short lived bombing campaign of the Germans against Britain, a longer bombing campaign by the British and Americans against the Germans, and the campaign of the Americans against the Japanese.
So what? The question is about strategic bombing and whether it had a “war winning” impact on either the war against Germany or the war against Japan.
I do not think any serious historian has argued that the Allied strategic bombing of Germany was “war winning”. It had an impact through direct destruction, diversion of resources, and damage to civilian morale but the war was won by the ground forces - primarily on the Eastern Front but with a significant input from Anglo-American (plus Canadians, Poles, French, etc) forces in the west.
The OP says “the campaign against Japan was a major success” but I still don’t think it was “war winning” prior to Hiroshima and Nagasaki. The war against Japan was primarily won by the US Navy once it started interdicting the supply routes to the Home Islands - the ground battles in Burma, the Philippines, and even China ground down Japanese forces but could not win the war and the air campaign was successful in inflicting massive damage on Japan but was not - by itself - a war winner .
Interestingly, much of the ‘effectiveness’ depended not on the quality of the planes, pilots, & bombs, but on the quality of their superior officers, especially in assigning targets and continued perseverance in hitting those targets over and over.
And in many cases, the quality of the commanding officers seems to have been lacking – on both sides.
The German bombing in the Battle of Britain started out targeting British Air Defense, hitting fighters, their airfields, etc. After a while, that defense was almost at a breaking point, when Goering/Hitler switched to bombing of British cities & civilians. While that was hard on the people, it had much less strategic effect, and gave the British Air defense a chance to rebuild. A big mistake by the German command; wasting much of the effectiveness of their bombing campaign.
The Allied bombing of Germany was also greatly reduced in effectiveness by decisions of the Allied commanding officers.
The British RAF did most of their bombing by night, to avoid German fighters and AA guns. But this meant much less control of where their bombs hit; they damaged whole ‘areas’ of civilian homes & businesses, but only rarely did major damage to their industrial/military targets. (You would think that the British would have realized the ineffectiveness of this when the Germans did it to them during the Battle of Britain.)
The US air forces had more ability to resist the German defenses (with heavily armed ‘Flying Fortress’ bombers & long-range (Mustang) fighter cover), so they did most of their bombing during the day, claiming ‘precision’ bombing. (Which wasn’t really all that precise.) But they failed in perseverance – as ftg noted, they kept switching to new targets instead of staying with and completely destroying critical ones. Probably a result of over-estimating the actual damage done by bombing raids.
When did the P-51 go into the European theater?
Wiki says they starting arriving in numbers in the winter of '43 - '44.
They began arriving during Fall of 1943 through Winter of 1944, replacing the p-38’s & P-47’s then in use. By about March of 1944, the majority of fighters were P-51’s, with the external fuel tanks that allowed them to accompany bombers all the way to Berlin and back. By the end of 1944, the fighter units were almost exclusively P-51’s.
Note that the Mustang had an exceptionally good reputation for speed and maneuverability. So much so that 20 years later, Ford Motor Co. picked that name for its new sportscar.
But they had to have British engines to achieve high performance. 
Or, so Wiki says, Packards built under license from Rolls.
Looks like they didn’t appear in large numbers until 1944. Before that, the bombers relied on the limited numbers of P-38 Lightnings (nicknamed the “Fork-tailed Devil” by the Germans) or just pure bravado and massed defensive fire to get through German defenses. Their casualty rates were… significant. During WWII, the USAAF lost more men in combat in either the European/Atlantic or Asian/Pacific theaters than the Marine Corps lost in the entire war combined.
On another note: Tranquilis mentioned in passing that the Eighth Air Force used incendiaries in Japan. The Eighth operated out of England during the war, while the Twentieth Air Force operated in Asia, along with the Fifth, Seventh, and Thirteenth Air Forces operating in the Pacific.
That’s hard to believe.
My father repaired B-25’s in the 5th.
Basically came down to numbers. There were a lot more Airmen than Marines in WWII (the Marines have always been the mean little runt of the Armed Forces I DIDN’T MEAN IT NOT IN THE FACE!) and they were exposed to much more widespread combat (recall that the air campaigns were going on pretty constantly in all theaters of the war from 1941 to 1945).
I just keep think up stuff about this thread …
There’s a variant of the chicken and the egg or some such metaphor going on with the differences between Germany and Japan. Germany had a much, much larger industrial infrastructure than Japan. So that meant there were a lot more targets to go after in Germany plus it also meant they Germans could crank out a lot more defensive equipment (planes, AA guns, etc.). Japan wasn’t in the same league as Germany or the US in terms of production capacity. So, the US had a smaller number of key targets to go after and an enemy that was going to have a much harder time producing enough gear to protect them.
Also, in Europe you had Albert Harris in charge of RAF bomber command and the US opinion of him is not good, to say the least. Typical out-of-touch British general. While Curtis LeMay was a couple bricks short of a full load, he was a genius compared to “Bomber” Harris.
Hey, give the P-47 its due!
“In Europe in the critical first three months of 1944, when the German aircraft industry and Berlin were heavily attacked, the P-47 shot down more German fighters than did the P-51 (570 out of 873), and shot down approximately 900 of the 1,983 claimed during the first six months of 1944. In Europe, Thunderbolts flew more sorties (423,435) than P-51s, P-38s and P-40s combined. Indeed, it was the P-47 which broke the back of the Luftwaffe in the critical period of January-May 1944.”
I love the Jug, but I was always under the impression that it just didn’t have the legs to protect the bombers on the longer-distance raids into Germany. Still, I always loved the very American solution to the problem of our fighter engines being underpowered. We made a fighter with a really big engine. 
Seriously, look at the size of the cockpit compared to the rest of the plane, then compare it to the Mustang. The Thunderbolt was a beast.
The U.S. has always been a fan of the addage that “more” and/or “bigger” is better.