Question: when did the army air corp become the air force
[{'answer': '20 June 1941', 'provenance': [{'wikipedia_id': '9895061', 'title': 'History of the United States Air Force', 'start_paragraph_id': 8, 'start_character': 71, 'end_paragraph_id': 8, 'end_character': 83, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]