From Wikipedia, so grain of salt and what not
Alexis de Tocqueville, 1831
The first reference to the concept by name, and possibly its origin, was by French writer Alexis de Tocqueville in his 1835/1840 work, Democracy in America:
The position of the Americans is therefore quite exceptional, and it may be believed that no democratic people will ever be placed in a similar one. Their strictly Puritanical origin, their exclusively commercial habits, even the country they inhabit, which seems to divert their minds from the pursuit of science, literature, and the arts, the proximity of Europe, which allows them to neglect these pursuits without relapsing into barbarism, a thousand special causes, of which I have only been able to point out the most important, have singularly concurred to fix the mind of the American upon purely practical objects. His passions, his wants, his education, and everything about him seem to unite in drawing the native of the United States earthward; his religion alone bids him turn, from time to time, a transient and distracted glance to heaven. Let us cease, then, to view all democratic nations under the example of the American people.
Seems like quite a different definition of the term than "Greatest force for good". In fact, almost all of the early references are about America having a unique set of circumstances when compared to European nations...America as fundamentally different, not fundamentally great.
