...- Boeing Co.'s cash outflows have been lower than expected so far in the first half of 2021 because of the acceleration of its 737 MAX deliveries and the lessening effects of the coronavirus pandemic on the demand for new aircraft and related aftermarket services. - However, there is still significant uncertainty around the pace of the recovery in global air travel and the company's 787 deliveries remain paused while it addresses manufacturing quality issues. - We affirmed our '###-/A-3' issuer credit rating on the aerospace and defense company. - The negative outlook reflects the possibility that Boeing's cash flow and credit ratios may not improve as we expect if it is unable to quickly resume its 787 deliveries, it delivers a lower number of MAX aircraft than we expect, or the recovery in global air travel stalls due to the appearance of new virus variants or a slowdown in vaccination rates....