Under Franklin Roosevelt's initiatives the United States modified it's post WWI neutrality laws to allow it to sell and transport goods to Britain prior to US involvement. Eventually US neutrality laws became so broad that even war materials were permitted to be sold, prior to the US entering the war.
The Merchant Marine Act of 1936 put U.S. federal dollars behind the annual construction of 50 U.S. merchant ships. That number was doubled in 1939 and doubled again to 200 ships by 1940. A lot of those ships were used to supply Britain prior to the US entering the War
In 1940 the British ordered 60 Ocean-class freighters from American Shipyards. These were used to replace lost merchantmen and boost their domestic merchant fleet.
March 1941, the British ordered and additional 200 merchant ships as part of Lend Lease authorized by the Defense Aid Supplemental Appropriations Act.
April of 1941 the British increased this order by an additional 106 ships totaling 366 ships 100 of these ships were the first Liberty ships delivered to the British.