The new C-12W is a King Air 350-based aircraft. The C-12Fs it’s replacing are based on the King Air 200.
Also see: MC-12W ‘The Flying Intelligence Shop’
UPDATE: Here’s a press release from Beecher Aircraft on the C-12W. It includes:
The current UC-12 aircraft are military transport versions of the Beechcraft King Air 200, which have been providing urgent intra-theater transport of high priority cargo/passengers to the USMC for the last 20 years. The UC-12W, a modified version of the King Air 350 equipped with a cargo door and military required equipment, is a modern and improved version of the UC-12.
As the flagship and the largest of the King Air line, the versatile King Air 350 has incredible capabilities and is ideal for military use. The UC-12W will provide the USMC with advanced technology and greater reliability. It is faster, has more range and carries more useful cargo and two additional passengers than the UC-12.