It's only taken 63 years but it looks like Frequency Modulation is coming the USA!
Currently AM and SSB modes are legal on the mid band 26.965 - 27.405, however documents recently published on the FCC website show that Cobra's 2017 petition to allow CB radio users to have FM has been successful:
CB radio equipment will still be required to operate on AM but will soon have the option to include FM if a manufacturer so wishes.
Of course, it remains to be seen if American operators embrace FM as an operating mode but I'm sure many people will welcome this long overdue decision.