The Cadillac of the Skies

I am extremely sceptical about this.

If you google "cadillac of the skies", the first seven or eight hits refer to Steven Spielberg's 1987 film version of J. G. Ballard's 1984 novel Empire of the Sun. In other words, this is a phrase made up by either Ballard or Spielberg (or Tom Stoppard, who wrote the screenplay), and not a nickname by which the Mustang was ever known in its day. (The ninth hit references a BBC documentary film that is no longer available, and almost certainly borrowed the phrase from Ballard and/or Spielberg.)

If further confirmation were needed, Wikipedia (always a sucker for a nickname) does not use the phrase, or even the word 'cadillac', in its North American P–51 Mustang page.

If the question had asked "Which Second World War aeroplane was referred to in Steven Spielberg's film Empire of the Sun as 'the Cadillac of the Skies'?", I would have let it go – even if, worded that way, it sounds more than a little obscure.