I've had a little paperback titled "No High Ground" since 1962, when it was published, which is a very detailed account of the Manhattan Project and the bombing mission, and that is where this answer comes from. Fletcher Knebel and Charles Bailey II are the authors.
There were actually seven B-29's involved in the first atomic bomb mission. Three flew together: the Enola Gay with the Little Boy; The Great Artiste, flown by Charles Sweeney, carrying instrumented packages to drop over the target and radio receivers and recording devices to measure blast pressures; and No. 91, flown by George Marquardt and carrying special camera equipment in place of the bombsight to photograph the detonation.
A fourth B-29, flown by Charles McKnight, flew to Iwo Jima where it waited to take over the Enola Gay's job if a mechanical failure made it impossible for the Enola Gay to complete it. The bomb would be transferred from the Enola Gay to the McKnight B-29, which was named Top Secret, and I believe is the airplane name Tom is looking for. The reason the Top Secret was flown to Iwo to wait was the risk on an explosion in the case of a landing accident, which it was expected would reduce Tinan to rubble. It was decided to risk Iwo instead. In order to protect Tinan further, as it held the only other ready bomb, Fat Man, the Little Boy device was, as a result of a last minute decision, armed after takeoff by Deac Parsons. The in flight arming was done after the Enola Gay levelled off at 4,000 feet on departure from Tinian and took 25 minutes.
The other three B-29's were weather aircraft and preceded the three primary aircraft to the possible targets. The B-29 of Claude Eatherly, Straight Flush (not the card game, the bathroom fixture) flew to Hiroshima. Ralph Taylor's Full House flew to Nagasaki, and John Wilson's Jabbit III went to Kokura Naval Arsenal.