From the Japanese websites I read, there are many caveats in the design. The range listed is one-way trip: it would dump its load over the US then proceed to land in German occupied France to refuel and rearm, then either bomb US again on the return trip too or go over the USSR, possibly bombing them too (as Luftwaffe ofc), maybe even India or China. Some claimed that the plane was supposed to utilize the stratosphere jet stream to maintain the range (like the balloon bombs), making their path predictable. The load was also too optimistic, and was said to be cut down to 15t later.
Most of the listed spec also relied on an improbable 6*5000hp engine (Ha-54) design, which was proven impossible due to cooling problems, one website claimed that the prototype literally caught fire during the test, and was downspeced to 6*3000hp engine (Ha-50). Three Ha-50 were made in or before 1944 but all "disappeared" following Japan's surrender, however one was dug up during Tokyo Haneda airport's expansion in 1984 and is now kept in The Museum of Aeronautical Sciences at Narita Airport, Google "ハ50" for images. It's badly rusted though and was thus not testable. There're many, many more problems with it before its cancellation, like having four wheels but two of which have to be jettisoned after taking off to reduce dead weight, making the "return bombing trip" implausible (unless Germany manufactures the wheels for Japan), and even then it's said that Japan couldn't make landing gears strong enough to carry the beast, and pressurized cabin remained an unresolved problem.
There's another problem with the project: we don't know the motive. It first emerged as a plan Nakajima Kikuhei (founder of Nakajima Aircraft) submitted to the military in 1942 titled "Invincible Air Defense Project" which called for giant bombers that can bomb the 50 states and can be remodelled as transport planes or attackers, but it wasn't until 1943/5/31 did the IJN general staff invited him to dinner for the said project, later the IJA also wanted this Wunderwaffe and the two ended up throwing competing requirements (F-35 anyone?), but that's not the point here.
The point is, that Nakajima was an outspoken critic against the decision of a war against the US, warning that Japan's industrial power was inferior to the US and they would not be able to overcome that. Also, Nakajima was probably not happy about the IRAA sidelining him, as he was an ambitious politician, having substantial influence in the Rikken Seiyūkai (one of the two major pre-war parties), and going as far as formally splitting his faction from the rest of the party (97 MP followed Nakajima, 66 stayed in the Orthodox faction, 10 stayed neutral, effectively tripartiting the Rikken Seiyūkai), though the chaotic Japanese politics ended in less than a year due to IRAA. In any case Nakajima was basically the last person you'd expect actively designing and lobbying for a Wunderwaffe, so there were a few conspiracy theories(?) out there, like he was actually aiming for the transport version for civilian use to keep Nakajima Aircraft afloat after the inevitable and appropriating military funds and resources in the process, or perhaps Nakajima's plan was to have his men be always working on something grandeur to please the military and prevent them from being drafted to die meaninglessly, and Fugaku was speced to be theoretically possible so that the military would buy into it, but technically implausible so no one could claim that they're slacking off while sparing them from draft indefinitely. These were further reinforced by the fact that the companies split off from Nakajima Aircraft mostly choose to put "Fuji" somewhere in their names (instead of something they actually completed like the jet Kikka which was a memorable feat), interpreted as people owing their fortunes to Fugaku. Later five of them would merge into Fuji Heavy Industries which is now known for its car brand Subaru. Nakajima would later be arrested on 45/12/2 by the Allied authorities for crime against peace before being released on 47/9/1.
