We heard back in February that Bullock was set to voice the film's leader, Scarlett Overkill; we now know (thanks to Deadline) that Hamm will be voicing her inventor husband, Herb. The updated synopsis for Minions is:
This spinoff establishes that Minions have existed since the beginning of time, in service of history’s most ambitiously evil villains. Through their lovable ineptitude, Minions have managed to thwart and ultimately destroy all of those bad guys. This story picks up in the 1960s, before they hench-ed for Gru. With no more super-villains to serve, the Minions fall into a collective depression and three of them set out to seek a new evil master. They wind up at a villain convention, where they compete for the right to be henchmen for Scarlet Overkill, whose Audrey Hepburn-like high style appearance belies her world domination ambition and her aspirations to become the world’s first female super-villain.
Directed by Pierre Coffin (Despicable Me) and Kyle Balda (The Lorax), Minions is currently set for a 19th December, 2014 release. Before then, Despicable Me 2 is on screens in the UK from 28th June, and in the US from 3rd July.