In an ambitious effort to shore up U.S. primary-care medicine, a coalition including General Electric Co., International Business Machines Corp. and Verizon Communications Inc. is launching an initiative to pay doctors hefty bonuses for creating "medical homes" for patients. The initiative is the latest and perhaps most far-reaching effort by Bridges to Excellence, a program backed by big employers and health plans to provide physicians with financial incentives for taking better care of patients. Last year, the program paid out roughly $10 million in bonuses to doctors in the 18 states where it is active.