Even now the double bass has not acquired a standard shape. Since its earliest days in the 16th century its shape has changed more than any other stringed instrument.
Nowadays two shapes exist side by side. One is like a violin (though of course much larger!) sometimes with a curved back, sometimes with a flat back (to make playing easier) and higher 'shoulders' attached to the neck.
The other is more like the shape of a viol with a flat back and 'shoulders' that slope more quickly and steeply away from the neck. More often than not the latter have sound-holes that are shaped like a backwards 'C'. The shape of the body resembles a pear.
There are even further examples of differently shaped double basses. One in the German National Museum in Nuremberg dates from 1563 and is shaped like a viol with many more additional curves in the body of the instrument than is found today.
Steve Williams of the Britten Sinfonia talks about the double bass he plays, demonstrating how long a double-bass can last in active use.
Introducing the double bass
A 'living' instrument?