DHL International Ltd. plans to announce an order for at least 34 Boeing 757s converted from passenger to cargo use, making the delivery company Boeing Co.'s first customer for the conversion program, people familiar with the order said. DHL, based in Redwood City, Calif., said only that it will spend $1.3 billion on planes for use in Europe and Africa, which would make it the biggest aircraft purchase in its 30-year history. DHL said it will disclose details of the order Tuesday. Boeing officials declined to comment. Boeing is buying from British Airways 34 used passenger 757s that it will refurbish as freighters, sources said. They said the Seattle-based plane maker may also buy as many as 10 passenger 757s elsewhere as part of the DHL order. By converting older passenger planes into cargo carriers, Boeing hopes to build a second market for them and increase their residual value. While brand-new 757s cost about $65 million each, the price of a converted 757 freighter would be less than half that. Boeing shares gained 25 cents to close at $42.88 on the NYSE.