How to change drag increments.

I am doing a simple summation task for 70 years of data. Each year has 12 things that need to be added together.

Is there a way I can write something like "= sum(B2:B13)", and then drag down so the boxes below look like:

"= sum(B2:B13)"
"= sum(B14:B25)"
"= sum(B26:B37)"
...

when I try this, excel increases every "B" by 1, I need to to increase the "b's" by 12.

Thanks