SAN FRANCISCO — Delta Air Lines Inc. and Northwest Airlines Corp. said Monday they’ve agreed to merge in a deal worth $17.7 billion, creating the world’s largest airline by traffic. Delta and Northwest said in a prepared statement that the all-stock transaction will create a combined company headquartered in Atlanta, and headed by current Delta Chief Executive Richard Anderson. Seven members of the new board will come from Delta, and five from Northwest, the companies said. The companies said the deal will create “a more resilient business model” better able to withstand high fuel prices and foreign competition. The deal is still subject to shareholder and regulatory approval, the companies said.