Note: in the current (2015) BJCP Style Guidelines, American IPA is in category 21 IPA From the guidelines: Category 21 IPA, is for modern American IPAs and their derivatives. This does not imply that English IPAs aren’t proper IPAs or that there isn’t a relationship between them. This is simply a method of grouping similar styles for competition purposes. English IPAs aregrouped with other English-derived beers, and the stronger Double IPA is grouped with stronger American beers. The term “IPA” is intentionally not spelled out as “India Pale Ale” since none of these beers historically went to India, and many aren’t pale. However, the term IPA has come to be a balance-defined style in modern craft beer.
The information below is still valid, but for those studying for the new BJCP exam, it may be incomplete. Use it as supplemental reading for the style and I will endeavor to update the styles as fast as I can.
American IPA, like most American versions of English styles, is bigger and bolder with a prodigious amount of citrusy, piney, resiny and fruity American hops. The style has been around for some time and many breweries brew and IPA in America. The style as we know it today, however, has only been around since the early 1970's. There are arguments as to which beer started the "American IPA" movement, but most will agree that it started with Ballantine's IPA, which some say was brewed just after prohibition was repealed. It had an ABV of 7.5% and an IBU level in the 60-70 IBU range. The beer was aged in oak for a full year giving the beer a wonderfully complex flavor. The current craze and popularity of American hop bombs may have began with Anchor's Liberty Ale which was originally brewed in 1975. Not long after, Grant's IPA (released in 1981) and Sierra Nevada's Celebration Ale (released in 1983) were released to meet America's growing appetite with bold hoppy beers. Americans were tired of the same old lagers and once we got a taste for these big beers, have not looked back.
Some versions of the American IPA are more traditional, more so than most English IPA's brewed today. They are somewhat pale with high hop bitterness that dominates the profile. These beers have a lot of hop aroma with low to medium hop flavor. More contemporary versions are similar in that they have high hop bitterness, but go one step further and add a big hop flavor as well as aroma. The American IPA's of today are dominated by the citrusy, piney, fruity and resiny hop character typical of Northwest American hops. The malt profile is generally low and clean with American 2-row the malt of choice. The use of Crystal malts is OK, but should be used with restraint.
To brew an American IPA, use a clean American yeast for best results, especially for competitions. These yeasts will give you the best attenuation and subdued ester profile the style demands. If you decide to use an English yeast instead of American, ferment at the lower end of the yeast's tolerance and substitute some sugar for the malt to make the beer more fermentable to allow the final gravity to come down. If you choose to use an English malt, such as Maris Otter, cut back on other malts such as Munich to reduce the overall malt character. I have had some luck in competitions with this style. But what I have found is that most judges prefer a fresher version of the beer, even though they may say something like "enter this beer again once it has had time to age". Most of the hop flavor and aroma will drop out with aging, and the judges can't seem to tell the difference between a well aged version and a version that was brewed with too little up-front hop character. These beers age well and can be a real pleasure to drink by the fireside, but as far as competitions go, I'd brew a fresh batch each year and enter it after 6 months of aging.
References: Information for this article was adapted from the 2008 BJCP Style Guidelines, and Brewing Classic Styles 80 Winning Recipes Anyone Can Brew, written by Jamil Zainasheff and John J. Palmer, and the Brewing in Styles section of Brewing Techniques magazine entitled India Pale Ale, Part II: The Sun Never Sets-- written by Thom Tomlinson.