6 x 12 header capability
Yes, a 6 x 12 Douglas Fir-Larch (#2 and better) with a 12’ span will support 8,000 lbs. at mid-span. It’s deflection will be less than 1/3 inch when fully loaded. 

If the 6 x 12 is a Select Structural, then it will support 10,300 lbs. and deflect about 1/4”. 

BTW, I recommend the window (vinyl, wood, whatever) that you are going to install below this beam should NOT be secured at the head in the middle one-half of the span.