The "Avenger"-class escort carriers were converted type C3 American merchant ships. Their design was based on the United States Navys (AVG1). To differentiate between the two classes, the Royal Navy ships were prefixed with a 'B' (BAVG). HMS "Dasher" (BAVG4) was built by the Sun Shipbuilding and Drydock Company. Originally named "Rio de Janeiro", she was laid down on 11 March 1940, launched on 12 April 1941 and delivered on 22 November 1941. She was converted to an escort aircraft carrier in the Tietjen & Long shipyards New Jersey and commissioned into the Royal Navy on 2 July 1942.