Not to pick an argument with Westie, but there were some archback hog-top D-25's made during the transition from flat back to archback, they're just kind of a rare bird.
D-25's are one of the most sought after Guilds, which is ironic because for quite a while it was their entry-level, low end model. I bought mine new, and it's still singing sweetly after 33 years
I think the arched back with no bracing makes it kind of a parabolic device that focuses all the sound to the sound hole. At least that's my theory, and I'm stickin' with it.
By the way, the arched back is made from laminated mahogany plys, no cheap stuff in the inner layers.
In the early 80's, the D-25 wasn't the sole model of this design, they came out with the D-15, 16, and 17, which were less expensive and consisted of an arched mahogany back and a solid mahogany top. I'm still hoping to add one of these to the stable