"The French geographer Roger Brunet, who wished to subdivide Europe into "active" and "passive" spaces, developed the concept of a West European "backbone" in 1989. He made reference to an urban corridor of industry and services stretching from northern England to northern Italy. Brunet did not see it as a new discovery, but as something easily predictable to anyone with "a little bit of intelligence and a feel for spatial properties."
He saw the Blue Banana as the development of historical precedents, e.g. known trade routes, or as the consequence of the accumulation of industrial capital. In his analysis, Brunet excluded the Paris urban area and other French conurbations because of the French economic insularity. His aim was a greater economic integration in Europe, but he felt that France had lost this connection in the 17th century. France, in his view, lost its links to the corridor as a result of its persecution of minorities (viz. the Huguenots) and excessive centralisation in Paris. Later versions do, however, include Paris.
Large population centres, e.g. Randstad, the Ruhr and Manchester, developed with the Industrial Revolution and further development would occur in areas that lay between these powerhouses."
Belts aren't strictly high population areas. It's really just the Sun Belt (where everyone with disposable income moved after WW2 with intensification in the 1970s during/after the oil crisis) and the Rust Belt (where they moved away from, except for African-Americans who tended to still move northward and get segregated by discriminatory housing policy).
We really just have one megalopolis- Mega-City One, er, Boswash. American population density is about half that of Europe, and we've got all this sparsely inhabited land west of the Mississippi all the way to California. I don't think this is a good comparison.