In 1964 City chairman Frank Johnson proposed the idea of the two clubs coming together, at a time when both sides got along well off the field and his side were suffering some of their darkest days.
There was a strong sense of goodwill in Manchester, with City allowing their rivals to use Maine Road while Old Trafford was rebuilt after being damaged during World War Two.
However, despite United suffering with the Munich air disaster of 1958 they had rebuilt and were gunning for the league title, which meant joining forces with City was an unattractive option.
Gary James has written a new history of City and spoke about what might have been.
"The idea was killed by both clubs before it ever became public," James explained. "I spoke to Eric Alexander whose dad Albert was chairman at the time, and he said Frank Johnson, who came up with the idea, often came up with crazy ideas.
"Another of his plans was to make the entire league regionalised into north and south.
"But City [under then manager George Poyser] were at a real low in their history at the time. In terms of league position, it wasn’t as bad as 1998-99, but in terms of general morale, atmosphere and support it was by far the lowest point in the club’s history."