user: Question: What is the aircraft for 21 crews?
Columns: ['Type of unit', 'Type of aircraft', 'Number of aircraft', 'Number of crews', 'Men per crew', 'Total personnel', 'Officers', 'Enlisted']
Types: ['text', 'text', 'text', 'text', 'text', 'real', 'real', 'real']
Sample row: ['Very heavy bombardment group', 'B-29', '45', '60', '11', 2078.0, 462.0, 1816.0]
SQL:

assistant: SELECT "Type of aircraft" FROM "Table" WHERE "Number of crews" = '21';