RAF and USAAF

Hi all,

I'm currently trying to find references to RAF personnel being 'loaned' to the USAAF, rather like USAAF individuals being seconded to RAF units. Exactly how often did the former happen (if at all) and is there any evidence to show that RAF aircrew went on combat missions as part of USAAF bomber crew?

Thanks for any leads anyone is able to offer.

Richard