There was a time John Dillinger was in jail. Legend had it he escaped using......

(1) He carved out a bar of soap to look like a gun. Used shoe polish to change the color.

(2) He carved out a potato to look like a gun. Put iodine on it thereby changing the color of the potato to look like a gun. A college chemistry teacher told us that one)

(3) A show on the History Channel told he carved out a wash board to look like a gun. Making his getaway.

So just what did John Dillinger use to escape with???

He claimed that he used a whittled wooden gun.
Link.

Seems I remember it as a potatoe carved like a gun.

All of the most reliable sites I've found say it's believed to be a wooden gun (blackened with shoe polish). I only found a few that said that he used a potato. Here's even more on the gun. That site also refutes the claim that he used a soap gun.