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Hard to say without more research. It is around 600 miles from the North coast of Hokkaido to Tokyo and around 1180 miles from the North coast of Hokkaido to the south (or southwest ) coast of the southern Island. So basically most of Japan was out of range of the B-29 even if it operated from the western most Aleutian Island. Trying to establish a major air base on a Russian Kuril Island (or Russian mainland?) not only faced opposition from Uncle Joe but the logistics would have been as bad or worse than using the more southern Pacific Islands.
From wiki so.......
"The chain has around 100 volcanoes, some 40 of which are active, and many hot springs andfumaroles. There is frequent seismic activity..."
"The climate on the islands is generally severe, with long, cold, stormy winters and short and notoriously foggy summers. The average annual precipitation is 30–40 inches (760–1,020 mm), most of which falls as snow."
Granted armies and AIr forces have operated in conditions as bad if not worse by why choose this option if there are others?
From an operational standpoint, I think B-29s on Attu would have been a disaster. Heavy weights, high landing speeds, short slippery runways with obstacles at the ends and no precision instrument approach system. Venturas and Harpoons are far more nimble airplanes than B-29s, which is hugely important if you have to fly a non-precision instrument approach in the fog into a short runway in mountainous terrain. Been there, done that. Piece of cake in a 1900 or a 99 or a King Air. Whole different animal in a Herc, a P-3, or any of the four engine piston pounders. It's all about weight, speed, turning radius, and climb gradient on the departure or the missed approach.
Here' are some airshow C-130 "aerobatics" at the Paris airshow in 2011. A very senior C-130 pilot:
"..... Yes, but to organize all the logistic chain, from a tiny island to another, without a Navy, I think that could have been for the Red Army an impossible task......
...... Not to point the fact that Red Army vast majority of tanks, artillery, ammo etc. in August 1945 was six or seven thousand km far from Vladivostok, with the Siberia in between."
Here are the numbers for the three fronts, Elmas:
Soviet forces under the Far East Command[1] Wikipedia
Totals (in bold)
Transbaikal Front
1st Far East Front
2nd FarEast Front
Men 1,577,725 (654,040 586,589 337,096)
Artillery pieces 27,086 (9,668 11,430 5,988)
Multiple rocket launchers 1,171 (583 516 72)
Tanks and self-propelled guns 5,556[c] (2, 416 1,860 1,280)
Aircraft 3,721 (1,324 1,137 1,260)
The resources for the operational were assembled in Siberia; Europe was only stripped of medical and engineering resources.
As for amphibious operations, this ...
"On August 18, several Soviet amphibious landings had been conducted ahead of the land advance: three in northern Korea, one in Sakhalin, and one in the Kuril Islands. This meant that, in Korea at least, there were already Soviet soldiers waiting for the troops coming overland. In Sakhalin and the Kurils, it meant a sudden establishment of Soviet sovereignty.
If Stalin had intended wider amphibious operations, Elmas, he would have assembled them by August 8, 1945.