The XB-15 was also designed - and built - and flown - as a bomber; so was the XB-19. Neither was developed into an operational aircraft; neither was the Me 264. So, functional equivalent.SpicyJuan wrote: ↑24 Apr 2021 22:59The Me 264 was not designed as a test aircraft, it was designed as a bomber from the outset and a limited series production was planned multiple times but cancelled due to limited German resources, the worsening war situation, and allied bombing.daveshoup2MD wrote: ↑24 Apr 2021 22:34In terms of a test aircraft that was used to try and figure out what a heavy bomber could or would do, what is the difference between the Me 264 program and the XB-15 program, in your opinion? Both were, essentially, operational test and design evaluation programs, and neither one resulted in an operational combat aircraft, much less an operational combat force. At least the Italians managed that with the P-108, albeit with very little success.
The Ju 390 was, essentially, the outgrowth of a modified transport aircraft, and demonstrates the design compromises such a project will always result in, given the significant differences between on and the other, as does the Japanese effort to turn out a strategic bomber based on the DC-4E airframe.
If you want to play that game, technically the Ju 390 was a further develop of the Ju 290, which itself was a development of the Ju 90 which was the development of the Ju 89, a bomber. This is all meaningless, however. The Ju 390 was significantly modified from the Ju 290 and was intended from the outset as a bomber.
In terms of paper designs, the American XB-20 - a modernized B-15 with more powerful engines - looks promising for its day, but it was supplanted by the B-29, which was more technically advanced than anything else developed in the same period. Again, functionally equivalent to the majority of German heavy bomber projects ... except for the fact the American designs that went into production in (roughly) 1935-45 - the B-17, B-24, B-29, B-32, and PB4Y - were all significantly more capable than anything the Germans came up with as contemporaries in the same period, most notably the He177. So were, for that matter, the Short Stirling, Handley-Page Halifax, and Avro Lancaster. So was the P108, for that matter.
Take any of the German designs that actually flew and put them in order by the year of the prototype's first flight. Then put their contemporaries in order by the same. Then compare, in terms of whatever criteria you value - bombload, range, speed, etc. Yuo'll have an answer.