Question: when did the us army air corps become the us air force
[{'answer': '18 September 1947', 'provenance': [{'wikipedia_id': '23869026', 'title': 'United States Army Air Corps', 'start_paragraph_id': 126, 'start_character': 49, 'end_paragraph_id': 126, 'end_character': 66, 'bleu_score': 1.0, 'section': 'Section::::Lineage of the United States Air Force.\n'}]}]