Sir Thomas Sopwith, the owner of Endeavour, was no stranger to the sea. He had raced in small boats for a long time and between 1927 and 1930 he won the Twelve Metre championships in Britain. Sopwith was familiar with the 'J' Class yachts as he had bought Shamrock V after Sir Thomas Lipton's death. Endeavour was designed by Charles Nicholson and built by Camper & Nicholson at Gosport. She was launched in 1934 with an overall length of 129½ft, a 22ft beam and with her steel hull, weighed in at 143 tons.

When Endeavour reached America, Sopwith arranged several races against Vanitie so as to get his challenger into shape for the forthcoming series. On board as regular crew were the designer, Nicholson, and Sopwith's wife. Endeavour won the first two races and Rainbow the third. Then there was the controversial fourth race, when Sopwith luffed and then bore away to avoid a collision that might well have endangered both crews. The committee's decision in favour of the defender marred an otherwise closely fought series.