 Anti-submarine armament consisted of two Squid anti-submarine mortars, while a quadruple set of 21-inch torpedo tubes were fitted.The ship was ordered as part of the Second Emergency Flotilla as "Packenham" on 2 October 1939, at a contract price of £416,770 (excluding government provided equipment such as armament), and was laid down at John Brown & Company's Clydebank shipyard on 1 July 1940.In early 1941, the ship swapped names with the destroyer "Onslow", also under construction, and was launched as "Onslow" on 31 March 1941.