Turbocharged recips don't suffer the huge low altitude fuel penalties that gas turbine aircraft do. A jet or turboprop B29 would have needed to climb to FL300 or higher for the long haul, even if attack altitude was 10K or less. Flying all the way at attack altitude would burn less fuel overall and keep them under the radar horizon much closer in to the target. That last 10K of climb in the thin air above 20K would be a long, slow, thirsty slog for heavily laden B29s.
On my way back from Aircraft Escape and Rescue Training at NAS Cecil, I hitched a jump seat ride in an Eastern DC9-10 from JAX to MIA with a couple of Navy trained Eastern pilots, both still active in the reserves, and both former "mustangs". Our flight profile (not the standard filed flight plan) was a max rate climb to FL260, 50 miles NE of Orlando, then a flight idle descent into MIA. It was late at night, we were high on the approach profile, traffic was dead, and the crew talked the controller (another ex-squid) into letting us do a high altitude military style teardrop penetration approach (not a peep from the cabin, just snores). The power levers stayed at idle until it was time to dirty up for landing. Fuel burn beat the programmed number for the standard FL180 flight plan by 15%, and we beat the scheduled flight time by 7 minutes. That's how dramatic the altitude difference is with turbines.
One of my buddies from the flying club taxied up to the gate as we were deplaning about 0100 (on to the tarmac; jetways all occupied by morning departures), and got me back to NQX in time to catch a few winks before morning muster.
Biff, Flieger, and any other active airline pilots out there, that was nearly fifty years ago, and rules were a tad different then. I even got my Commercial and CFI without an Instrument Rating at first. Had to upgrade in 1975.
Cheers,
Wes