Complex sentence: The air echelon squadrons of the 7th were en route to the Philippines and arrived at Pearl Harbor on the morning of Dec 7 1941 and consisted of the: 9th, 463rd, 492nd, and 493d Heavy Bombardment Squadrons.
Split into simpler sentences: The air echelon squadrons of the 7th were en-route to the Philippines and arrived in Pearl Harbor on the morning of Dec 7 1941. They consisted of 9th, 463rd, 492nd, and 493d Heavy Bombardment Squadrons.