US operated spitfires did cover some B17 escort duties up to March 43, when they were taken off 'offensive' operations, and conversion started to the P47.
There are other records of them escorting RAF Boston raids, again prior to 03/43. They didn't have the legs to do the full long range escort, but certainly did into occupied territories.
PS. Google often isn't your friend at all, and wicki is far, far, worse.
