the noun 'Boston' is a proper noun, as it is a specific place name. In addition to the city in Massachusetts, it could also refer to Boston Harbor or Boston Bay in Australia, as well as any of the several townships around the world that bear this name.
1. I'm flying to Boston this weekend for a business trip.
2. Have you ever been to Boston Harbor?
3. The Boston Township in Pennsylvania has a population of 1,367.
the noun form of 'Boston' should be capitalized to distinguish between the proper noun of the city and other usages of the word.