Apple is expecting the iPhone 3G to be a big success with customers and may be planning on cranking out over 40 million of the combination iPod and smartphone by the end of August 2009. Sources close to Apple claim the plan to build so many handhelds was made after the iPhone 3G launch proved to be more successful than anticipated, according to BusinessWeek.

If the source is correct, Apple is planning on topping earlier production estimates by about 50 percent based on Piper Jaffray analyst Gene Munster’s 26 million unit estimate. It also exceeds Apple’s original plan to build 30 million iPhone 3G handhelds in the same time period.

Apple launched the iPhone 3G on July 11 in over 20 countries, and plans to roll out the handheld in an additional 21 countries on August 11. The company hopes to be selling the iPhone 3G in about 70 countries by the end of 2008.

Assuming Apple can produce the 150,000 units five days a week that BusinessWeek’s source claimed, it will be able to build 39 million handhelds in a 52 week period, bringing it inline with production estimates.

There are, however, some stumbling blocks to selling 40 million iPhones that quickly. Apple needs to finish its multi-country roll out to expand its market area, and it also needs to address stability and performance issues many users have been complaining about.

Reports of poor 3G performance have been dogging the company since the phone’s U.S. launch in July, and now one user is hoping to gain class-action status for a lawsuit that alleges the iPhone 3G is defective.

If Apple can manage to keep its iPhone 3G customers happy while avoiding the distractions of drawn out lawsuits, it may well be able to sell every iPhone it produces.