There are (at least) 3 bands by that name.
The first one existed from 1975 to 1977
Widowmaker were a British hard rock group, who were active from 1975 to 1977. They were considered by many to be a supergroup and released two albums. Although their influences appeared to offer vast creative possibilities, musical and personality differences lead to their break-up. The legacy of Widowmaker is captured on the compilation Straight Faced Fighters (2002) released by Castle, which contains tracks from both of their albums.

Read more about Widowmaker on Last.fm.