Why do Boeing planes start with 7?

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Tags

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2. #rockets
3. #engines
4. #missiles
5. #decided
6. #model
7. #numbers
8. #turbine
9. #began
10. #transport
11. #aircraft
12. #commercial
13. #aside
14. #ended
15. #represented
16. #reserved
17. #military
18. #would