SINGAPORE/PARIS (Reuters) - Singapore Airlines (SIA) announced a provisional order worth $13.8 billion at list prices to buy 39 Boeing wide-body planes on Thursday as it battles Gulf carriers, dealing a blow to Airbus in the key market for big twinjets. SIA signed a letter of intent with Boeing for 20 777-9s and 19 787-10s to tap additional passenger growth and to modernize its fleet over the next decade. Additional options for the two aircraft types could take the deal to as many as 51 aircraft.