In early 1939, with the risk of war with Nazi Germany increasing, it was clear to the Royal Navy that it needed more escort ships to counter the threat from Kriegsmarine U-boats.

To meet this requirement, the Smiths Dock Company of Middlesbrough, a specialist in the design and build of fishing vessels, offered a development of its 700-ton, 16 knots (18 mph; 30 km/h) whale catcher Southern Pride.

Apart from providing a very useful space where the whole crew could gather out of the weather, the added weight improved the ships' stability and speed and was retroactively applied to a number of the original Flower-class vessels during the mid and latter years of the war.

Petunia was one of 30 Flower-class corvettes ordered on 31 August 1939, and was laid down by Henry Robb at their shipyard at Leith on 4 December 1939 and launched on 19 September 1940.

[4] On 5 July 1941, Petunia, together with the corvettes Lavender and Starwort and the Admiralty yacht (and former survey ship) Challenger, were escorting the troopship Anselm Gibraltar when the convoy was spotted by the German submarine U-96.

Petunia then dropped a depth charge to discourage further attack, the noise of which causing Barbarogo's captain, Enzo Grossi, to claim to have sunk his target, which he identified as a Mississippi-class battleship.

[6] Petunia was then allocated to the forces preparing for the Invasion of France,[6] and on 6 June, together with the corvettes Clarkia and Pink, the trawler Northern Foam and a Motor Launch, escorted assault convoy J.15, with 12 LSTs to Juno Beach for the landings.

[6] On 19 March 1947, Fu Bo collided with the steamship Haimin at the Wuqiuyu lighthouse outside the mouth of Meizhou Bay, Fujian Province, sinking the corvette.