My vote for the slowest bomber would be the Po-2/U-2
Originally built as a transport/utility, it was used by the Russians in vast numbers as a light night harrasment bomber, it was usually fitted with a pair of 120kg bombs or (occasionally) 4 RS-82 rockets.
Flown by the famous 'night witches', female combat pilots in VVS service, the Russians had a serise of nicknames for it, including 'lawnmover', 'grasscutter' and 'sewing machine'.
The Po-2 was powered by a 5 cylinder radial that produced barely more than 100 hp. Night bombing runs would often be made with the engine switched off, gliding in towards the target area, and then releasing the bombs onto the unsuspecting enemy position.
And the maximum speed? Just 155 kph/98mph!
Reportedly, when a Po-2 was spotted they were quite difficult to shoot down. The planes were so slow and nimble that they drove German pilots in the faster and far more powerful 190s and 109s into fits just trying to engage a target doing less than 1/3 their velocity.