SHANGHAI—Just ahead of U.S. President Donald Trump’s visit to China, U.S. aviation regulators signed a new pact with their Chinese counterparts that effectively opens the door to selling China-made airplanes and parts to the U.S. and other countries.

The agreement boosts Beijing’s hopes of eventually becoming a global supplier of commercial jetliners and of breaking into a market currently dominated by Boeing Co. and Airbus SE.

The...