The first requirement, and the most pressing, was a new flagship for the destroyer flotillas. The Japanese naval designers sat down and looked at the Agano class to see what could be done with it. The first and obvious candidate was the ship's armament. The 5.9 inch L40 mountings were a miserable effort, probably the worst of their kind deployed anywhere. The guns themselves were long obsolete British-designed weapons originally supplied as secondary armament on Kongo class battlecruisers. The gun armament would have to be changed; there were, quite simply, no more guns of that type available. In any case, a quick examination showed that they were inferior to the 3.9 inch L65 as deployed on the Type B destroyers in virtually every category.
That line of thought also led the designers to take a closer look at the Type B destroyers. Although intended for anti-aircraft deployment, they were too small and presented too unstable a gun platform for the role. The Type Bs had excellent gunpower and were fine destroyers but anti-aircraft ships they were not. A true anti-aircraft ship would require a larger hull in order to provide better fire control and battle management capability. The new light cruiser being developed was an obvious candidate for this role. The only problem was that the 3.9 inch L65 seemed very light for a ship of these dimensions.
The only alternative was the twin 5 inch L50 as installed on the existing Type A and C destroyers. The problem was that limited train rates and elevation made its efficiency in the anti-aircraft role dubious. Better guns, the Type 1 and Type 5 5 inch guns also existed but they were shore-based anti-aircraft weapons. They would require entirely new mountings and the delay was not acceptable. In the end, the designers accepted that they had little real choice. They elected to take the eight 3.9 inch armament of the Type B destroyer and install it on the Agano hull. The new armament was not only more effective than the previous mix of 5.9 inch and 3.9 inch guns but it used space much more efficiently and freed up the center section of the ship. The design team used this area to install a heavy battery of torpedo tubes, four quintuple mounts, two mounts on each beam. No reload torpedoes were provided, the tubes on the unengaged side being considered the equivalent to a reload set.
In terms of propulsion, the designers adopted the 80,000 shp plant of the Type C destroyer for the new ship. This was intended to be the standard power train for the Japanese Navy, the intention being that one unit driving two screws would be used for destroyers and light cruisers while two units totaling 160,000 shp driving four screws would be used for heavy cruisers and larger ships.
Another major change related to protection. The Agano had a conventional armor scheme for a ship of its size, a limited 3 inch belt and a two inch deck. The designers recognized that this was inadequate in the face of modern weapons but also that it was impossible to provide adequate protection on a ship of this size. By deleting the armor completely, the ship would be significantly lighter, much easier to build and not place so many demands on scarce armor production capacity. It was probably at this point that the designers realized they had ceased to design a small light cruiser and started to produce a big destroyer instead.
In fact, the improved Agano class turned out to be a very attractive design. The more efficient machinery, revised armament and the deletion of protection started a virtuous circle that eventually shaved over 1,600 tons from the displacement while using the same basic hull. They had a reasonable (by 1945 standards) anti-aircraft capability, excellent anti-ship firepower and they were large enough to carry a substantial anti-submarine armament, six depth charge throwers and two racks with a total of 60 depth charges. They were a very impressive multi-role destroyer that could fulfill all the roles of the existing Type A, B and C destroyers and those of the proposed light cruiser. Their anomalous nature was reflected in their service nomenclature. Officially they were listed as being cruisers but their design folder clearly designed them as being the "Type D Multirole Destroyer"
The four existing Aganos were rebuilt to the new design. They were essentially an intermediate type retaining the older style machinery and armored protection but equipped with the new armament. The four conversions were followed by 16 newly built ships, which represented the full implementation of the modernized design. These replaced the older light cruisers on a one-for-one basis, starting with the Tenryu and Tatsuta. The improved Agano class therefore became known as the Tenryu class. They still retained an odd combination of cruiser and destroyer characteristics, the most notable being the catapult amidships for a single seaplane. By 1948 when the first Tenryu class destroyer entered service, that was already an archaic as well as an anomalous feature. After the first eight ships, it was deleted and the area used for boat stowage. The catapult was subsequently removed from the rest of the class and replaced by ship's boats.