It looks very much like a B-17 Flying Fortress, one of the main four-engined bombers used by the USAAC. Contrast your GF's GF photo to this one Unless the picture is reversed, they are on the starboard side of the plane . The black thing in the upper-center is a deicing boot, but unfortunately I cannot make out what variant of B-17 it is as the photo's composition obscures the main differences between B-17 models (tailfins, waist gun positions, nose, belly guns, etc.).

Because of the low wing, and four engine configuration, I would hazard it is a B-17. not much else to go on, unless we know the location or bomber group it was assigned to. Any more information you could provide?

Although I fully agree with the two other responses thus far on the B-17 basis (note the oval-rectangle-oval on the starboard wing between the two nacelles here http://www.earthtothemoon.com/wof_zg.jpg ) that there are only five men in the picture may indicate that the aircraft is not a B-17 with its ten-man crew, but perhaps a variant such as the C-108 or F-9. Unless that's the maintenance crew for the aircraft, which is always possible. Was GF's GF an aviation mechanic or ordnanceman?

Just some information on a way it was deduced that it was a B-17 variant. First, the fact of two engines on one side of the fuselage quickly allows you to reduce the number of possible aircraft down to three base models, the B-17, B-29, B-24, because these were the three primary four-engined planes used by the USAAC. Second, you can discard the B-24 from the pool as it has a high wing which would've come way over the people's heads. Third, you can get rid of the B-29 as it has a clean leading edge on its wings between engines. Leaving you with a B-17 or one of its variants. Like the others said that's about all we can tell from that one photo.