Sept 29 (Reuters) - Boeing Co (BA.N) said on Wednesday the U.S. Department of Defense had awarded it a follow-on contract valued at up to $23.8 billion to provide services to a fleet of C-17 Globemaster III transport aircraft over a 10-year period. Under the agreement, Boeing will continue to provide services including engineering, field support and material management for a fleet of 275 aircraft managed by the United States Air Force and eight global partners, the planemaker said. Boeing will also lower operating cost per-flight-hour for the fleet under the new agreement, it said. The new agreement is currently funded through September 2024, the company said, with a Phase I award of $3.5 billion.