If my ever fading memory recalls, the first attempt at torpedoing with the B-26 was at Battle of Midway and it was a failure. 
Partially correct.  There are only two instances the USAAF used B-26s as a torpedo bomber and both actions occured on the same day.  While the B-26 torpedo bombers were attacking the IJN fleet off Midway, another group of B-26 torpedo bombers were attacking the IJN carrier Ryujo off the Aleutians.  In both attacks, the B-26s suffered heavy losses and didn't hit anything.
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