I don't think there is actually a bassline for 12 bar blues, but I took a standard blues bassline and transcribed it to the guitar chords. You just play this and improvise. You can get some sweet solos out of this. Just remember to keep your improv. in the right key. 
Note: each progression is over two bars, though it seems to be written in six-bar. 
Thanks to jacksloadedgun for pointing that out
   E               A                   B to A               Ending E
G| ------------------------0-------    ----------------     ------------
D| --------0-----------2-4---4-2---    --1-4-1-----2---     --------0-2-
A| ----2-4---4-2-4-0-4-----------4-    2-------0-4---4-     ----2-4-----
E| 0-4-----------------------------    ----------------     0-4---------            

Swing the notes, try playing two to a note. Just make something up.

If you want you can bend the C# to D and F# to G instead of just playing the open note.
It sounds cool.

I usually play it E E A E halfB halfA endingE

Have fun, you can keep the soloing up for hours if you want to. I've rated this advanced because, despite the fact that the line is easy, soloing and improv. take skill.