Apple Inc. (NASDAQ:AAPL) is preparing for the biggest iPhone launch in the company's history having asked Asian suppliers to manufacture the upcoming 4.7-inch and 5.5-inch versions of the iPhone 6 by Dec. 30, with combined orders for both models totaling between 70 million and 80 million units.

The launch of the new iPhone 6 models is expected to be huge for Apple, as it is estimated to represent a sizable jump over last year’s initial orders of between 50 million and 60 million units for the iPhone 5s and iPhone 5c, the Wall Street Journal reported, adding that the new iPhone 6 models with larger screens are expected to draw in consumers now used to seeing bigger handsets from Samsung Electronics Co Ltd (KRX:005935), Sony Corp (TYO:6758) and other Asian phone makers.

According to the Journal, both models of the iPhone 6 could feature metal cases similar to the ones found on the iPhone 5S and are likely to come in multiple colors. However, Apple is reportedly facing problems with the production of the larger screens as suppliers are struggling to ramp up production of the 5.5-inch displays, the Journal reported, citing people familiar with the matter.

Apple also has reportedly asked its suppliers to manufacture components for up to 120 million iPhones by the end of 2014 to compensate for possible display failures. Last year, the company had asked suppliers to be prepared with components for 90 million iPhones.

The Journal also cited Michael Walkley, an analyst at Canaccord Genuity, as saying that there is “strong pent-up demand” for the iPhone 6 as customers have held off on upgrading from older iPhone models in anticipation of a larger iPhone, which is likely to be released sometime in mid-September.

Apple is expected to launch bigger iPhones at a time when Android-run handsets with larger screens have become increasingly common in the high-end segment of the smartphone market. In May, 98 percent of Android smartphones that sold globally for $400 or more featured a screen larger than 5 inches, according to Counterpoint Research.

Meanwhile, both Foxconn Technology Co Ltd (TPE:2354) and Pegatron Corporation (TPE:4938) are expected to begin production of the 4.7-inch iPhone 6 in August, while production of the 5.5-inch version is expected to begin in September, the Journal reported.