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.
   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———0—4———————————4—    2———————0—4———4—     ————2—4—————
E| 0—4———————————4—————————————————    ————————————————     0—4—————————

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

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.

Version History
Version 1.0—That took little effort
Version 2.0—A little bit too little effort, so I rewrote it, it's a bit more fun.