As MDPilot said many are graduates of one of the six test pilot schools. This most certainly does not mean ALL of them are. The fact that Boeing Commercial Aircraft are principally operated by civilians leaves the door cracked slightly. Some are ex-military without test pilot specific training, others are strictly civilian. A few are Boeing engineers who lived a double life flying on the side and then transitioned to the flight test position when the time was right. What all of these folks have in common is a strong engineering background coupled with diverse flying experience. Here is a sample test pilot job posting on Boeing's website to give you an idea:
Occupation Title Flight Operations-Pilot
Job Title Transportation Production Test Pilot 3
Skills Management Title Production Transport
Level EXEMPT
Business Component Commercial Airplanes (BCA)
Experience Level Experienced
Job Location Seattle, WA
US Person Status Required? Yes
Security Clearance No Security Clearance Required
Security Requirements
Job Description Serves as a captain and independently performs production flight tests. Consults with engineering and manufacturing to define and refine requirements for production programs and individual tests, including the nature and extent of tests required and the order in which production aircraft functional tests are conducted. Executes all production maneuvers required to determine flight or system characteristics, aircraft performance, and structural integrity to determine compliance with design specification. Demonstrates safe conduct and efficient use of flight time, and documents comprehensive production flight test results. Interfaces with functional engineering and manufacturing on project teams to ensure consistency in overall operability of aircraft or systems. Prepares production flight reports on aircraft engineering or production functional problems. Confers with engineering and manufacturing personnel to analyze aircraft or systems performance, aircraft handling qualities, and equipment malfunction. Plans and develops technical reports and provides input for developing and modifying flight manuals and related documents. Represents the flight operations organization to senior management and internal customers, and represents the company to external customers. Pursues continuous process improvement.
Competencies Require a Federal Aviation Administration (FAA) Airline Transport Pilot (ATP) or commercial pilot's license and appropriate category endorsements. Require a minimum of 1,500 hours pilot flight time and passing a physical and pilot performance screening.
Education Require an engineering degree or equivalent education and experience. Require 2 to 5 years' company or related industry experience in addition to job family entry requirements.
Relocation Money Available YES
Notes An advanced degree is desirable. Prior production experience on Boeing commercial airplanes is highly desirable. Test pilot school graduate is highly desirable.
As you can see the perfect candidate will be a TPS graduate with a master's or higher in an engineering field and time in type. Once again, however, realize there are current Boeing test pilots that do not meet all of these requirements. Best of luck to you!