There's a recorded instance of a B29 shooting down 14 fighters over Tokyo (it was also rammed by two fighters and still made it home). There are also instances of B29s downing MiGs over Korea.

The late model B17s were pretty effective too. An old Strategy & Tactics magazine quoted stats for two different air forces (recall it was the "US Army Air Corps" and "Air Forces" were simply an organizational unit) operating out of Britain over a two year period in WWII, one was bombers and the other fighters. Both shot down roughtly the same number of planes -- but the bombers used 100x as much ammunition. (Sorry, this is a 1970s print publication, no link, just my memory.)

Also recall that the Japanese lost almost all their trained pilots early in the war, so the B29 was using a far superior weapon system against very inexperienced pilots.