Favorite plane never built (or perhaps as a prototype).

[aqhoffman]

Ditto on the B-35 Flying Wing, but for late war deployment in the Pacific where its range and payload would exceed that of the B-29. Initial contracts specified late 1943 delivery of the first prototype. WW2 B-35 production might have been possible if this project had a higher priority, perhaps to augment and be the immediate successor of the B-29 with the B-32 cancelled?

However, my other favorite that never made it into full production is the Republic XF-12 Rainbow. Republic XF-12 Rainbow - WikipediaView attachment 764064

I was literally coming into this thread to post this answer (the XF-12 that is). What a beauty.

This a great documentary on it.
View: https://www.youtube.com/watch?v=ZTy0zyfP6KI

 

[GrauGeist]

MAIN reason Concorde/Tu144/Boeing SST failed was economic, pure and simple. It couldn't be justified even as a corporate/national image booster.

Ozone was just the media whipping boy of the day, with spray cans, refining plants, and conifer forests all sharing the blame in the activist movements.

The effects the SSTs had on the upper atmosphere was actually a real thing.

Aside from the public hand-wringing over the ozone issue and sonic booms, NASA and the ESA were taking note of it to the point of conducting some pretty serious studies.

Here's a PDF report from NASA on the subject:

https://ntrs.nasa.gov/api/citations/20205009400/downloads/CR-20205009400.pdf