Trapnell was the first US Navy pilot to fly a jet aircraft, was considered the best, most experienced naval test aviator of his generation, co-founded the branch's first test pilot school, and played a pivotal role in both the development of future Naval aircraft and the survival of the post-World War II Navy's air arm.

His father and several cousins attended the United States Naval Academy in Annapolis, Maryland—although Benjamin's military career was cut short by an infamous hazing incident.

Several other cousins were officers in the United States Army as was his brother, Wallace Probasco Trapnell, who served in the Signal Corps.

The team was disbanded in April 1931, and Trapnell was soon assigned to the small plane unit attached to the Navy's dirigible airfleet.

In 1938, Lt. Trapnell flew in a squadron of eighteen bombers from San Diego, California, to Honolulu, Hawaii, in what was "the greatest over-ocean formation flight" to date.

He initiated a series of lectures and classes to familiarize pilots not only with the rudiments of flying but to learn the intimate details of flight engineering, performance, stability, and control.

"[3] Trapnell later gained valuable knowledge of what the Hellcat and its predecessor, the Wildcat, were up against after performing extensive tests in a captured Zero recovered from a crash that same year.

Following Trapnell's recommendations after months of testing, engineers at Vought Aviation extensively redesigned a new fighter already under development, the famous F4U Corsair.

On April 21, he became the first naval aviator to pilot a jet aircraft, the Bell XP-59A Airacomet, the first such plane built in the United States.

In October 1944, he became chief of staff to RADM Arthur Radford, Commander Carrier Division Six, overseeing the remaining air strikes and amphibious landings in the Pacific theater.

He also received the Octave Chanute Award that summer for "showing outstanding ability not only in flying every type of aircraft but also in detecting critical defects in new airplanes and suggesting ways to deal with them.

[8] In October of that year, Trapnell appeared before the House Armed Services Committee during the Revolt of the Admirals incident, where he provided crucial testimony on behalf of naval aviation.

Among other innovations, Trapnell revised the system and apparatus utilized for carrier take-offs, considerably streamlining the amount of time expended for the procedure.

[11] Both marriages were to women of socially prominent West Coast families; the Trapnells made several appearances in articles covering socialite circles usually involving festivities in Coronado, an affluent city near San Diego.

During a banquet given by former residents of Charles Town, WV at the St. Denis Hotel in New York, he caused a public stir when he lambasted the McKinley Administration's policies in regards to newly acquired territory from the recently concluded Spanish–American War.