What was the original "Plan B"?

I mean, when, where and how did the term Plan B enter the common vernacular?

And once we’ve established that, what was the original Plan A?

Cane had pizza for two ordered if the whole kill Able thing fell through.

According to the on-line form of the OED, Plan B is cited as in use in a literal sense as early as 1863, in what seems to be a discussion of a military plan, in comparison with a Plan A in 1870 (involving steel costs), and in the colloquial sense in 1977 (in a Washington Post article).

“Let there be . . . uh . . . more dark.”

Oops. Thought this was going to be about that old “douching with Coke” birth control legend. Carry on.