According to Kelly Johnson's A-12 Log, on 14 Sept 1960 they
started work on a bomber version. This saw light as SP-229,
"RB-12 Proposal" on 15 Dec 1960. I believe that Byrnes &
Hurley's "Blackbird Rising" mentions this, but I'm away from
home and my copy, so I can't check.

In 1965, they revisited the concept as the B-71, but like the
earlier ones, it was never built. I've seen drawings of three
variants with bombs in the chines (like the YF-12's missles),
a rotary bomb dispenser in the fuselage (inside the covers
of Crickmore's books?), and a single large bomb in the fuselage.

But as an earlier writer has posted, the B-12 was never built
in any form.