You don't even need to include ICBMs.

In the Vietnam war, the US dropped about 3.5x the number of TNT tonnage equivalent bombs than was used in WW2.

Likewise both Iraq wars exceeded WW2 tonnage, even if you include the 2 nukes dropped on Japan.

Part of this is due to advancements in conventional explosives (lighter explosives with more power than TNT) and larger conventional weapons and increased capacity of bombers. Not sure how many MOABs were used in the 2nd Iraq war, but it was more than one, and theyre about a 20k ton TNT equivalent (more powerful than the little boy nuke).

I dont have any handy citations, but the data is easily googled or on wikipedia.

You'd need to include the infrastructure to be able to launch B-29 fleets over Tokyo, which was much of the Pacific Theatre effort. That itself was a major share of the total $288 billion (1945 dollars) US expense of the war itself, far more than the Manhatten Project itself.

Mainland US flights were unable to reach Japan, even from Hawaii or Alaska. Large-scale effort required control of a large portion of the Pacific and both launch and recovery capabilities.

A significant number of aircraft for the Doolittle raids (early in the war) were ditched before they could reach their designated LZs in China. The aircraft, B25B Mitchell medium bombers, could launch from carriers (easier to arrange for than island occupation), but lacked either the range or raw capability to land on the Hornet. The raids were effective as a shock and demonstration of US reach and capabilities, but had minimal direct military effect.

Winning the war required control of a large portion of the Pacific - it's not like we were doing it just to give B-29s a place to park. The atomic bomb required the same thing, as it was B-29 carried just like incendiaries.

Supporting a single aircraft, or a small flight of seven, is vastly different from the hundreds of aircraft engaged on the Tokyo firebombing raids:

https://en.wikipedia.org/wiki/Bombing_of_Tokyo

Dresden required nearly 1,300 bombers.

https://en.wikipedia.org/wiki/Bombing_of_Dresden_in_World_Wa...

The ultimately produced bombs were deliverable via B-29, as that aircraft was available. The carrier-launched Mitchell B25B was also available and had a payload capacity of 3,000 lb., though that is 1/3 the weight of Little Boy (the Hiroshima bomb). Whether or not a smaller package, or alternative delivery vehicle, could have been provided, I'm not sure. Though the challenge might have been surmountable.

https://en.wikipedia.org/wiki/North_American_B-25_Mitchell

https://en.m.wikipedia.org/wiki/Little_Boy

Casualties per mission over the European theatre were tremendous. I don't recall the specific per-mission losses, but overall crew survival over a 25 mission tour was on the order of 50%. Japan's defences were less effective, but again, looking at use of atomic weapons generally, the statistics are relevant, and during the planning of the Manhattan Project, use of the bomb against Germany was considered.

http://www.ww2f.com/threads/bomber-crew-survival-rates-durin...

There's also the possibility that smaller flights might escape detection (smaller radar target).