The corporation shall have a board of directors (hereinafter in this section referred to as the “board”), consisting of fifteen members. Three members of the board shall be appointed by the President of the United States, by and with the advice and consent of the Senate, effective on the date on which the other members are elected, and for terms of three years or until their successors have been appointed and have qualified, except that the first three members of the board so appointed shall continue in office for terms of one, two, and three years, respectively, and any member so appointed to fill a vacancy shall be appointed only for the unexpired term of the director whom he succeeds. Twelve members of the board shall be elected by the stockholders.