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. One particular concern was the need to protect shipping off the east coast of Britain. What was needed was something larger and faster than trawlers, but still cheap enough to be built in large numbers, preferably at small merchant shipyards, as larger yards were already busy. To meet this requirement, the Smiths Dock Company of South Bank -on-Tees, a specialist in the design and build of fishing vessels, offered a development of its 700-ton, 16 knots (18 mph; 30 km/h) whaler (whale catcher) Southern Pride. They were intended as small convoy escort ships that could be produced quickly and cheaply in large numbers. Despite naval planners' intentions that they be deployed for coastal convoys, their long range meant that they became the mainstay of Mid-Ocean Escort Force convoy protection during the first half of the war.

The Flower class became an essential resource for North Atlantic convoy protection until larger vessels such as destroyer escorts and frigates could be produced in sufficient quantities. The simple design of the Flower class using parts and techniques (scantlings) common to merchant shipping meant they could be constructed in small commercial shipyards all over the United Kingdom and Canada, where larger (or more sophisticated) warships could not be built. Additionally, the use of commercial triple expansion machinery instead of steam turbines meant the largely Royal Naval Reserve and Royal Naval Volunteer Reserve crews that were manning the corvettes would be familiar with their operation.

Flower-class vessels were slow for a warship, with maximum speed of 16 kn (18 mph; 30 km/h). They were also very lightly armed as they were intended solely for anti-submarine warfare; many of the RCN's original Flower-class ships were initially fitted with minesweeping equipment, while virtually all of the modified Flowers were fitted with a limited anti-aircraft capability. The original Flowers had the standard RN layout, consisting of a raised forecastle, a well deck, then the bridge or wheelhouse and a continuous deck running aft. The crew quarters were in the forecastle while the galley was at the rear, making for poor messing arrangements. The modified Flowers saw the forecastle extended aft past the bridge to the aft end of the funnel, a variation known as the "long forecastle" design. 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 applied to a number of the original Flower-class vessels during the mid and latter years of the war. The original Flowers had a mast located immediately forward the bridge, a notable exception to naval practice at that time. Th…

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The RN ordered 145 Flower-class corvettes in 1939, the first 26 on 25 July with a further batch of 30 on 31 August, all under the 1939 Pre-War Programme. Following the outbreak of World War II, the British Admiralty ordered another 20 on 19 September (all from Harland & Wolff) under the 1939 War Programme. This was followed by an order for a further ten Flower-class corvettes from other British shipbuilders two days later. Another 18 were ordered on 12 December and two on 15 December, again from British shipbuilders. The RN ordered the last ten vessels (under the 1939 War Programme) from Canadian shipbuilders in January 1940.

By the end of January 1940, 116 ships were building or on order to this initial design. The ten vessels ordered from Canadian shipbuilders were transferred to the RCN upon completion. Another four vessels were ordered at Smiths Dock Company for the French Navy, the first ship being completed for the Free French Naval Forces in mid-1940 and the other three being taken over by the RN. Another 31 Flowers were ordered by the RN under the 1940 War Programme but six of these (ordered from Harland & Wolff) were cancelled on 23 January 1941.

The RN ordered 27 modified Flower-class corvettes under the 1941 and 1942 War Programmes. British shipbuilders were contracted to build seven of these vessels under the 1941 Programme and five vessels under the 1942 Programme; two vessels (one from each year's Programme) were later cancelled. The RN ordered fifteen modified Flowers from Canadian shipyards under the 1941 programme; eight of these were transferred to the USN under reverse Lend-Lease.

The RCN ordered seventy original and 34 modified Flower-class vessels from Canadian shipbuilders. The Canadian shipbuilders also built seven original Flowers ordered by the USN, which were transferred to the RN under the Lend-Lease Programme upon completion, because wartime shipbuilding production in the United States had reached the level where the USN could dispense with vessels it had ordered in Canada. The RCN vessels had several design variations from their RN counterparts: the "bandstand", where the aft pom-pom gun was mounted, was moved to the rear of the superstructure; the galley was also moved forward, immediately abaft the engine room.

Shortly after the outbreak of war the French Navy ordered 18 Flower-class vessels, 12 from UK yards, two from Ateliers et Chantiers de France at Dunkirk and four from Ateliers et Chantiers de Penhoët at Saint-Nazaire. The two At. & Ch. de France ships are listed as "cancelled" but the four Penhoët ships were under construction at the time of the Fall of France and …

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The original Flower class were fitted with a 4-inch (102 mm) gun on the bow, depth charge racks carrying 40 charges on the stern, a minesweeping winch and a 2-pounder (40 mm) pom-pom gun on a "bandstand" over the engine room. Due to shortages, a pair of Lewis guns or quadruple Vickers HMG was sometimes substituted for the pom-pom, which would have left the ship very vulnerable to aircraft attack in its envisaged role of coastal convoy escort and patrol in the North Sea. The long-range endurance of the vessels, coupled with early war-time shortages of larger escort warships, saw Flowers assigned to trans-Atlantic convoy escort where Luftwaffe aircraft were rarely encountered. Vessels assigned to the Mediterranean Sea usually had more anti-aircraft guns fitted. Underwater detection capability was provided by a fixed ASDIC dome; this was later modified to be retractable. Subsequent inventions such as the High Frequency Radio Direction Finder (Huff-Duff) were later added, along with various radar systems (such as the Type 271), which proved particularly effective in low-visibility conditions in the North Atlantic.

The Flower class had been designed for inshore patrol and harbour anti-submarine defence and many required minor modifications when the Allied navies began to use them as trans-Atlantic convoy escorts. These small warships could be supported by any small dockyard or naval station, so many ships came to have a variety of weapons systems and design modifications depending upon when and where they were refitted; there is really no such thing as a 'standard Flower-class corvette'

Several of the major changes that vessels in the class underwent are indicated below, in a typical chronological order:
• Original twin mast configuration changed to single mast in front of the bridge, then moved behind the bridge for improved visibility.
• Heavy minesweeping gear removed for deep-sea escort work and to improve range.
• Galley relocated from the stern to midships.
• Extra depth charge storage racks were fitted at the stern. Later, more depth charges stowed along walkways.
• Hedgehog fitted to enable remote attacks while keeping ASDIC contact.
• Surface radar fitted in a "lantern" housing on the bridge.
• Forecastle lengthened to midships to provide more accommodation and better seaworthiness. Several vessels were given a "three-quarters length" extension.
• Increased flare at the bow. This and the above modification created the modified Flower design for subsequent orders.

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