Final answer:
The Swing Era bassist known for using substitute chords, melodic solos, speed, flexibility, and an attractive timbre, who played mostly with the Duke Ellington band during 1939-1941, is Jimmy Blanton.
Step-by-step explanation:
The Swing Era bassist being described is Jimmy Blanton. Blanton revolutionized the role of the double bass in jazz with his innovative approach to the instrument. He used substitute chords and his solos were characterized by their free melodic nature, contrasting the typical walking bass lines of his time. His ability to play with speed and flexibility, coupled with his attractive timbre, set a new standard for bass players. Jimmy Blanton’s remarkable career, although short-lived from 1939 to 1941, was mostly spent with the Duke Ellington band, where he made a significant impact on the development of jazz.