The characters are interesting, the plot is well developed, and I liked it. At one point I thought I had caught Mr. Gann in an historical contradiction. However - yes - Boeing did introduce the pressurized model 307 Stratoliner before Pearl Harbor.
It's not his best book, but certainly well worth a read. I preferred "The High and the Mighty" in aviation fiction, and "The Antagonists" and/or "Twilight for the Gods" in his non-flying fiction.
A minor (but obtrusive) point: My copy (Arbor House, 1983) suffers considerably from electronic spell-checking as a substitute for proofreading - in which the wrong correctly-spelled word is frequently substituted for a misspelled (or unrecognized) word.