The Americans will gather on Tuesday in Miami to prepare for matches on 3 June at Costa Rica, where no United States team has ever won, and against Honduras, third in the regional qualifying table, at Chicago on 6 June.
"Clearly these matches are very important in our efforts to qualify," said USA coach Bob Bradley. "We are certainly aware of the challenges of playing in Costa Rica and Honduras has produced some great results."
Howard will report after playing for Everton against Chelsea in England's FA Cup final on Saturday while DaMarcus Beasley and Maurice Edu of Rangers report after next weekend's Scottish FA Cup final against Falkirk.
The unbeaten Americans lead the CONCACAF qualifying table with seven points, one ahead of Costa Rica, whose home match was moved ahead a week so the USA team could prepare for June's FIFA Confederations Cup in South Africa.
The three top teams in the six-team round-robin tournament will book spots at South Africa 2010, while the fourth-place nation will meet a South American team for another berth.
Seven starters from the past three USA qualifiers are on call, with all-time scoring leader Landon Donovan setting the pace in experience with 108 caps.
Jozy Altidore, who scored three goals in a 3-0 victory last month over Trinidad and Tobago, and Michael Bradley, who scored twice in a 2-0 victory over Mexico, lead USA scorers.