Humility as an American Virtue

Thinking about George M. Cohan's patriotic song, "You're a Grand Old flag."  The line that sticks out is:

Every heart beats true 'neath the red, white, and blue
where there's never a boast nor a brag

In a song extolling the virtues of the nation, humility is set out as key.  Nationalism seems to be antithetical to humility, and nationalism is certainly not the same as patriotism although the two are often confused for each other.  At the same time, humility seems not to be a virtue we currently celebrate.

So, when did humility cease to be an American virtue?  Was it ever?