After the delivery of the 65th aircraft, a further change was worked into the production lines, which introduced folding wingtips to allow them to fit on aircraft carriers. The resulting Model 21 would become one of the most produced versions early in the war. A feature was the improved range with 140 US gal wing tank and 85 US gal drop tank. When the lines switched to updated models, 740 Model 21s had been completed by Mitsubishi.