Originally Posted by
Zorak
Right -- to clarify further, it is true that when you first input a search, if you leave the destination blank and go to the map view, Google Flights only lets you use a very small subset of the available filtering criteria; for example as jsloan says you can (initially) only filter by alliance and not by individual airline.
But once you enter a destination (any destination), that opens up additional criteria, such as specific airline(s) as Smiley90 said. And, to spell this out further, once you have selected your more-specific criteria, you can delete the destination to go back to the map view but with your more-specific criteria intact.
I'm gonna step in and be the "um actually" guy here, but you can filter by individual airlines in the map view. You need to look at the URL and replace "a:STAR_ALLIANCE*STAR_ALLIANCE" with "a:UA*UA"
The URL is a great way to modify filters beyond what google lets you, e.g. you can pick your exact max price instead of having to use the options provided on the slider.