As stars of the picture, Brian Donlevy was named after weeks of careful consideration to play General Groves. Army chief of the atomic project, and Robert Walker. fresh from his portrayal of the late Jerome Kern in "Till the Clouds Roll By," was selected to appear as the general's aide.   Tom Drake, following his spiritually keyed performance in "The Green Y ears," was decided upon as the ideal type to symbolize Arnerica's youthful scientists on the screen. Beverly Tyler, introduced so successfully as Drake's sweetheart in the previous picture. was chosen as his young bride, experiencing the hardships suffered by the wives and families of the men on America's top secret undertaking.   Audrey Totter, another brilliant young actress, won the role of Jean O'Leary, General Groves' confidential secretary who later ,.was decorated by Congress as a woman who knew the secret - and kept it. Hume Cronyn , was picked for the important Dr. Oppenheimer role, .Joseph Calleia as Professor Fermi,. Jonathan Hale as Dr. Bush,. Henry O'Neill as General Farrell, Warner Anderson as Admiral Parsons (a Navy captain at the time depicted in the film), and Barry Nelson as Colonel Paul W. Tibbets, Jr., pilot of the B-29 that dropped the atom bomb on Hiroshima.   England and Canada were represented in their participation in the project by Hurd Hatfield and Richard Haydn, two of Hollywood's most distinguished performers, in the roles of the British scientists called to America on atomic research, and Damien O'Flynn. as C. D. Howe, Canadian Minister of Munitions.   In its effort to tell the full story, Hollywood sought to present the project not only as an all-out, all-American development, but also as the perfectly coordinated Allied triumph that it represents.   "The bomb position is designated zero"