They were all built by contract with commercial builders to a common design prepared by Jacob Acworth, the Surveyor of the Navy; however, there were small differences between individual vessels, with a platform deck being constructed in the hold in Swallow (i), Merlin, Raven and Swallow (ii), whereas the other seventeen had no platform deck and thus their depth in hold was thus nearly twice as much.

The Merlin class design was one of two standard designs to which all Royal Navy sloops were built between 1743 and 1748 (the other design being the Hind class designed by Acworth's colleague Joseph Allin), who was appointed to share the post of Surveyor with Acworth on 11 July 1745.

Although initially each was armed with ten 6-pounder guns, this class was built with seven pairs of gunports on the upper deck, enabling them to be re-armed with fourteen 6-pounders later in their careers.

The first two – Swallow and Merlin – were ordered on 7 July 1743 to be built to replace two ex-Spanish vessels (the Galgo and Peregrine's Prize, both captured in 1742, and put into service by the British).

On 5 April 1745 five more were ordered – including a second Falcon (named to replace the first, captured in the same year) and a second Swallow (similarly to replace the first, wrecked in 1744) – and a single extra vessel was ordered on 11 April.