Instructions: In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Input: Passage: In 1943, in preparation for overseas deployment, 400 pilots and 3,600 crewmen of the 351st Bombardment Group assemble at a Colorado air base. Arriving in England, as part of the Eighth Air Force, the 351st BG is welcomed by the Royal Air Force and begins training, attending daily lectures on security. Their first mission flying Boeing B-17 Flying Fortress bombers has the armada departing at 15-second intervals.
The first mission of the 351st on Fortress Europe, after fighting through Luftwaffe defenders, is a success, with all 20 B-17s returning safely back to base. The danger of flying in massed formation, however, is brought home in training, when a collision takes place over the home base. Missions are also not always without casualties and 351st bombers are shot down.  Wounded airmen who make it back to base, are cared for at a nearby hospital. When bombers are all shot up, the ground crew works up to 90-hour periods to put them back into service.
As the mission tallies build, the men partake in activities at the base, including games, socializing and worship. The U.S.O. brings entertainers such as Bob Hope while the Red Cross provides the men with rest and recreation. During special parades, commanders of the 8th Air Force bestow medals for exceptional service and sacrifice.
The 351st participates in a big mission into Germany and after flying for about three hours, the B-17s' fighter escort turns back. As the bombers near their target to drop incendiary and anti-personnel bombs, enemy fighter aircraft appear and intense flak comes up from the ground. After a long battle, the 351st returns to base, ready to fight again until the war is over.
Output:
What is the formal name for the group that departs at 15-second intervals on their first mission?