The J/K/N-classes were a return to smaller destroyers after the experience of the larger Tribal class that preceded them and which prioritized gun armament over torpedoes. These three classes carried a total of ten torpedo tubes, more than any other British wartime destroyer class. Gun armament consisted of six 4.7-in QF Mk. XII guns in three twin mountings which were identical to those on the Tribals. An option for a 4-in QF Mk. V HA gun was also provided in place of one of the torpedo mounts. Anti-air armament included a quadruple 2-pdr pompom mount, with up to four single Bofors and 10 20-mm guns added by the end of the war. The three flotillas saw extensive service during the early war years and consequently suffered appalling casualties: only two of the J and K flotillas survived the war, with all but one of the losses being in the Mediterranean. Surprisingly, however, only one N-class ships was lost. Five of the N-class ships were transferred to Australia, another two to the Netherlands (one of them later transferred to Indonesia) and a sole ship to the Polish Navy. All remaining Royal Navy ships were out of service shortly after the war. Perhaps the most important legacy of the J/K/N-classes was that their hull and machinery served as the basis for the War Emergency Programme destroyers, of which over one hundred were built.

Most of the ships served early in the war with the Home Fleet where some participated in the Norwegian campaign, the Dunkirk evacuation, and escort duties. Most were then transferred to the Mediterranean where they suffered appalling losses from numerous causes, primarily air attack. One, HMS Janus was sunk by a Fritz X missile off Anzio. The only loss in a different theater was HMS Jupiter which was sunk in the Battle of the Java Sea after accidentally hitting a Dutch mine.